meh. kind of of a chicken and egg question/answer, but there it goes:
qt5 is not supported by SUIL, nor the other way round; so no qt5 ui embedding is ever possible, not even in ardour3;
qt5 plugins are not in-process loadable into qt4 hosts (also, having SUIL to load them just makes any host a qt4-like one, so it will crash as bad), and vice-versa;
that said, qt5 ui plugins will possibly work either on a qt5 host or else if both implement the lv2_external_ui and/or lv2_idle/show_interfaces (the former is deprecated while the later are the recommended form nowadays);
not much documentation about all this, but you can sort the vee-ones, and qtractor of course, as they implement all of the above scenarios;
meh. kind of of a chicken and egg question/answer, but there it goes:
good luck!
cheers