I, too have to jump in.
I agree with Jack that the thermal problem may not exist,( as far as the
spokes acting as heat sinks and resulting in a cold solder joint.)
On RF designs, we sometimes have 0805's mounted to external planes
with no thermals whatsoever. We have not encountered any
solderability issues by doing this.( I am not recommending this, just
including it as a reference).
The important thing is that you solder mask over bare copper, leaving
only the pads with tin-lead on them.
What happens, is that if the mask is over tin-lead, the tin-lead will migrate
during reflow, and the solder will go with it.

Gary