first try would be to add "psv" access everyhre where there's already
yes|permissive|designated|destination (details vary)
then add some logic to also include "bus=yes" and "psv=yes" tagging into the access logic
So that you end up in en expression like:
assign caraccess
= or bus=yes or psv=yes
switch motorcar=
<... see car-vario.brf ..>
motorcar=yes|permissive|designated|destination|psv
Only a good idea to reproduce existing urban bus-routes