The trick is to add " -m32" to the "Compiler name" in build methods. This switches the compiler (and linker) to 32-bit mode.
A little bit harder part is installing missing libraries. For this to happen, make sure to add "/usr/lib32" to your Libraries paths in build method and then perhaps the best is to install U++ on 32-bit system (e.g. another partition) and to copy missing libraries from it. Tedious process, but should work. (I must admit I tested only with console app I need for work).
Whether you can package 32-bit .debs on 64-bit system is just another question