The main problem is the poster. 'I think' is defeatist. It rarely rains so hard, for so long, at this time of the year that all beekeeping is suspended for a whole day. Exactly how long does it take to pour the bees into a hive? Where would these bees be if they were not collected and had not yet re-homed themselves?
I would recommend the method of:
Wait for a lull in the downpour.
Remove roof and crownboard of recipient hive.
Remove a couple or three central frames.
Transfer bees to box ensuring all are out of the swarm box.
Replace crownboard and wait about 5 mins.
Replace frames (removed previously) taking care not to crush bees.
Replace crownboard and roof.
Job done.
If the bees are already on frames, it is just as easy, if not easier!
No real problem at all, as far as I can see. Anyone who thinks the worst, before even looking out the window, needs to think carefully about how bees have got by for millions of years.