Well, Big Jim your in luck as I understand what a lawyer says not what they mean.
I guess this is why I am considered an Expert witness for several Manhattan law firms.
OK lets explain what your asking to the folks.
Your looking to modify your hydronics system (hot water heating) and you closed the fill and return lines and NOW your concerned about residue water in the system especially now that it is isolated you cannot drain this system as your creating a vacuum, which you were correct in assuming.
To solder the joints properly and cut in a new section YES you are correct in opening a few bleeder valves to remove ALL the water in this zone so you can solder a DRY PIPE safely following all applicable codes and safety precautions and any and all other escape clauses etc.
It would also be a great idea to shut the automatic filler to your boiler AFTER you shut your boiler off electrically as dry firing is a bad thing.
While your system is down it would be a fantastic time to consider installing a low water cut off.
Personally giving any lawyer advice is really taking a chance BUT stating the obvious and being an expert I will consider this a one time professional courtesy thing.
UNLESS you send me a private E mail then I can explain more in more detail as I would hate for the non knowing P&H frauds out there to try to do something and get themselves hurt by taking something out of context.