In my experience with many different types of hacks (Urals, Yamaha 650/Velorex, old '75 Goldwing/Velorex, HD Nightster/Cozy) - this is a general rule for me:
Max (or near) pressure on the tires - all three.
And that is based on the manufacturer's max pressure on the sidewall.
The sidecar is a load on the bike - and as with any large load, to increase tire pressure is a good thing.
And that can be very different - my Cozy pressure is 33 psi - with the Metzler - but *not* so with the stock Indian tire that came with it - and now serves as the spare. (see
http://mysite.verizon.net/respud62/id10.html)