is there a "Heat off" option with your wifi?
wire your old mercury bulb tstat in paralell, set it to your low temp. turn off the wifi stat until you want to heat higher
scooter bum got me thinking wire your mechanical stat in parrallel with wifi stat. Mech stat set at whatever leave wifi stat off can look at room temp and if you want to raise the set point just turn on wifi stat
Ding ding ding ding ..Winners! Here's what happens when you overthink something - I'd gone through all sorts of permutations for fooling the new stat, probably thought about paralleling at some point and rejected, because it just didn't seem right

. Now that it's been suggested to me, it makes sense, shut the heat off on the new 'stat, I still get monitoring even at the lower setting of the mech 'stat, when I want higher heat, turn the heat on on the new 'stat, it'll override the mech one, which will be off any time it's above it's set point..
BTW - yes, the system does have a C wire, so the new 'stat is happy..
Thanks for all the responses!