Can't think of anything more linear than those infernal (lowercase :P) boat maps.
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The cargo ship maps are literally a straight line if you pick the right one. Though, bit of a warning, if you have collection tasks, the items can spawn on top of shipping crates, which is a bit annoying.
Im using a hero sewer map for something just like that. I forget the number off the top of my head. It has one front room, then 2 medium sized sqaure ones in the middle, and then a small back room. soo far it has worked nicely. check out my arc if you like, Just hit the first mission and see if its something you could use. I have it set sorta what your looking for. a hostange in first, hostage in middle and hostage and boss in end mission.
PS if ya wanna give me some feed back id welcome it.. I know clues need a major over haul and dialoge i think needs some more work on flow but over all im happy with the way its turnning out.
GL !
i looked for ya.. its Large, hero seweres 15. its not very big despite it being catagorized large.
I would recomend you cut the spawn to 3 though, as the middle can spawn in either room, if you put 2 in there you may risk the chance of it spawning in the back room then the front room.. but going with 3 in F/m/B works flawless
What I think I might do is put in a single glowie in the back, and make the boss spawn contingent on the glowie rather than one of the rescues. This way, I can keep each of my rescues and make them independent of the boss trigger.
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How about if you use Rescue 1 to spawn Rescue 2...then Rescue 2 to spawn Rescue 3...then have Rescue 3 spawn the Boss. It might be a little strange since Rescue 2 and 3 might be in places the person has already looked, but it could possibly work.
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If it's doable within your storyline, almost all of the Longbow Set of tech maps are VERY linear.
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I've done this in "Quid Pro Quo."
Hostage 1: spawns.
Hostage 2: spawns when hostage 1 is complete.
Boss: spawns when Hostage 2 is complete.
If you had a linear map (such as a cargo ship) with a defined Front, Middle and Back set of spawn points you could cascade the rescues one after the other and, unless the player chooses to stealth all the way to the end, chances are good he wouldn't notice.
It's theoretically possible to do so with more in a row than I did, but I wouldn't really advise it on most maps, because there's no telling which order the player will find the hostages in. On a map large enough to contain 4 hostages, chances are good the player will bypass one or more of them, have to backtrack, get frustrated, and wonder why you're so cruel. 
It might work with Escorts. Some players like to pick up Escorts as if they're luggage and carry them around with 'em, gathering all the Escorts before returning to the exit, but I think you can make something spawn trigger upon the rescue of an escort, and another spawn trigger upon the completion of the Escort.
I have a couple similar approaches to missions, Heraclea. Warehouse maps are pretty good for this, and actually some of the office maps are fairly "linear" (even if they have multiple floors). Some of the cave maps are linear, too, though they can curve around a bit. Look around in them and see what works is about the best I can say, in addition to looking at these suggested.
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Depends on theme and size for what you want. But the big thing is to look for the maps with 1 or 2 back.
I haven't tried these, other than encountering them and they may all work:
Caves[*]5th Column HQ[*]Council Giant Robot 01
Office[*]Skulls
Unique[*]Circle of Thorns - Spider City (huge)[*]Widow Tower (the most linear)
The trouble with this is, when it comes to glowie placement, the game often ignores the front/middle/back directive and places them at random. Either that or some maps have _very_ loose definitions of front and back.
I think each map has, let's say 6 spawn points for each setting. That's 6 for Front, 6 for Back and 6 for Middle, and Middle shares 1 or 2 spawn points with Front and Back, perhaps with the exception of very large maps, so there's always a chance your stuff will spawn much closer to the front or the back then you intend.
Which I suppose is a good thing. If the single Front spawn was always in that same place on that map, it'd get mighty dull.
What I'd like to do is to have a boss that spawns only when a series of four previous rescues has been completed.
I can't do that at present. The best I can do is to put the last rescue last in the listing, specify it as "back", specify the boss as "back", and set the boss to spawn after the back rescue has been completed. This way, hopefully the boss will spawn after everything else has been finished, in the same room as the last rescue.
What are the most linear maps I can use? The ones where it's almost certain that the player will have rescued everything before the final rescue and boss? I don't want to send the player looking for one less than final rescue after she's already beaten the evil mastermind here.
Anyone have any specific map suggestions? I know Infernal works fairly well here, but it isn't what I am looking for. I tried several of the tech maps, but they too often have forking paths and multiple floors and elevators.
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"At what point do we say, 'You're mucking with our myths'?" - Harlan Ellison