I don't know about vacant channel allocations around here, but we have two stations that have noncomm and commercial subchannels already, WPAN and WFBD. Both carry TCT along with commercial supported networks, so I suppose PBS could lease space on one or both.
So far as I am aware, TCT affiliates are treated by the FCC as commercial stations, even if they run no commercial advertisements. As to asking for "love offerings" for merchandise, that runs into a gray area. Public TV stations do likewise.