Two questions:
The situation is: Here we are sitting in a cave and all the bad guys have to do is wait for sunset to kill us off before we can raise our parmas again. I realise it is too late for this time, but...
... would it be possible to have a diameter parma spell to bridge that gap?
No. Sun spells end at sunrise/sunset. Diameter spells last for a whole diameter regardless of when you cast them.
A diameter parma is not something that you see, generally. I would say that Parma is a kind of Sun duration spell, so no diameter version. Unless you work around alternative parma rules (like stockade parma et al) as they appear in HoH:TL.
IMS we allow magi to put their parmas in circles: you basically create a circle/ring parma and lose your parma yourself (tyou are parma-less if you leave the circle). It lasts indefinitely as long as trhe circle is not broken and/or you do not recast your parma.
Now, the best effect in your situation might be to cast nice pyrotechnic effects to confuse the hell out of the enemy and earn precious time to recast your parmae. Invisibility spells, chaotic imaginem spells, teleport.... all those would help, I guess.
Or you could try to negotiate with the guys instead of getting the heavy artillery out. Or use your grogs as meat shields. Literally. MuCo is nice for that.
As an alternative, you could try to cast a diameter duration WoMS or a ReVi effect to divet incoming spells. Or you simply attack before being attacked yourselves. If at sunrise/sunset there is no enemy to attack you, what is the problem?
Cheers,
Xavi
Note that the bad guys can't exploit this weakness if they don't know how parma magica works.
Since this is the order best-kept secret, this seems very unlikely.