LEGO glue or sealant

An interesting problem. While the group choice (rec.toys.lego) reflects your application, I can't help but wonder if there might be a more fruitful location to ask this question.
The usual answer to the first requirement is in direct conflict with the second. Oatey brand plumbing adhesive, sold to glue bits of PVC pipe together, bonds many other plastics very well. I keep a jar around for repairing kids toys. I don't let the kids play with them for a day or so after gluing, as the fumes are nasty. It works by melting the plastic together, which is a bit of an overlap with "eat[ing] ABS".
Let's look at the requirements: clear, glossy, water soluble, slow dry, cheap, low fumes. All of those could be met by simple syrup (ie sugar and water or corn syrup and water). That wouldn't be a strong glue, and long term it would probably collect dust on exposed areas, but it might do the trick for you.
I don't quite understand what you are asking for in the bullet point begining "sprayable...". Are you asking for something to coat the outside of the bricks to hold them together post assembly? That's not really a job for glue...
Elijah ------ simple syrup is sprayable, but might clog a sprayer

Thank you for the response.
After MUCH experimentation (I've tried spraying and reg. glueing Elmer's Washable Glue AND Gel, Elmer's Glue All, and Aleene's Tacky Glue) I've basically come to the conclusion that it's not possible to glue the outside. Yes, I was hoping to find a clear coating that would both hold the model and save time reassembling.
These glue's work well (the best is Aleene's Tacky) and result in a nice coating if brushed on and a 'medium' hold. A medium hold being that a child cannot pull them apart, but an adult can, it takes some force. The result is a clear coating (no cloudiness) that comes off easily when soaked in a bath tub of warm water. It's almost completely smooth and not noticable with the same gloss as the original bricks, when brushed properly.
An interesting note is that applying the glue on the outside actually results in these glues working into the seams between the bricks somewhat. It's not a perfect seal
When it comes to spraying these glues however they need to be watered down so much (like a milk consistancy) that the result is a runny mess on the non-pourous, slick bricks. The glue just runs off and pools at the corners. When it dries it's uneven and bumpy. So much for the 'dream' of spraying something on this. I must have been crazy to think that will work.
At this point, I think it is unavoidable to glue each brick in place to maintain the original finish... adding alot more work to the projects. That said, I am experimenting with Elmer's Washable Glue versus Aleene's Tacky Glue, glueing a test model together and waiting an entire week (despite the instructions, the actual full cure time for these is a week as with most PVA glue, because of the semi-seal the bricks make and the air pocket inside it takes much longer for these to dry).
I will then do some strength tests. I think the result will be a strong-to-medium hold (stronger than just sprayed) that can be broken by 'adult' hands if needed at the week points. The model is so large it will not fit in a bathtub without breaking into chunks.
The final test will be then to soak the brick chunks in a tub to see if water can get into the seams such that it will fall-apart and the glue will desolve completely, recovering the bricks easily.
Note that Elmer's Washable has GREAT reversabilty but has a POOR hold on plastics. Aleene's is GOOD solubility but has GOOD hold.
I hope I'm not rambling and some day this post'll help someone. I'll follow-up with the best option I find.
Thanks!

Thanks for the write up. Exactly what I wanted to know. My son has a Lego set he wants to us as more of a toy and I want to help him get there. Thanks again!

Thanks for the write up. Exactly what I wanted to know. My son has a Lego set he wants to us as more of a toy and I want to help him get there. Thanks again!