Isometric games are really something I love to play around with, particularly grid-based ones.
What you're thinking would probably be very difficult to make with Construct, however I can see how it could be done (with lots of improvisation).
I will probably attempt an isometric game sometime, however right now I think I'll try to write a grid data parser (ie, it takes a string of 4,4,2,5,6,2,65,7,3,1,6 into a grid of tiles identified by ID).
After that we'd need to make some form of grid to isometric stuff, basically getting the right points to attach the grid tiles to on their XY coordinates (and maybe Z coordinates), and the character movement could be dealt with using a custom movement (always move x distance towards ('newx'),('newy') + Key Up Arrow is Pressed -> Add 1 to ('gridy'), set ('newy') to <work out some crazy "the grid space is here in pixels" stuff>)... after that it'd be a case of making walls and stuff attach to the grid spaces where appropriate, and then making a game.
Alternatively, we could simply use 3D boxes and sprited characters with even a stationary camera, but... y'know, that'd be easier. And much less cool, and you wouldn't really be able to brag about it much because Construct did way too much of the work.