So, today I had some time to tinker with FvwmPiazza. First and foremost I must admit Thomas was right, it's an awesome module and it should be encouraged to use.
There are a couple of quirks - one needs to install Base::Class from CPAN and point fvwm at the installed Piazza with ModulePath, but other than that it's plain awesome. But one thing I'd like to do is undecorating tiled windows. Since layouts are assigned on per-desktop basis, per-desktop Style would be great. Is there any way to do that?