I've been thinking (uh oh. . .) and this idea of having Thranduil come to the rescue of the Lorien Elves is perhaps not such a good one.
First, there's no mention made anywhere of Thranduil marching out of Mirkwood -- or even a sizeable army of his having to do so. It also doesn't make much sense -- I mean, Lorien is guarded by Galadriel and Nenya: if anything, they're in a better position than Thranduil. Finally, Thranduil's kingdom is a long long long way away, so for there to be time for him to send aid, there would have to be an army of his already heading south through Mirkwood for some reason.
So how about this: no Mirkwood army, but a scouting party. They spot a large army of orcs and easterlings heading for Lorien. They then free the Lorien envoy (which could have some females
), then the freed Envoy and the Mirkwood scouting party have to try and race back to Lorien ahead of the easterling/orc army to warn of the attack.
Lots of potential for plot there I think: evade the orcs and easterlings, get across the Anduin (with the only fords held against them?); before the freeing of the envoy, there could be lots of interaction between the captured Elves and the orcs and Easterlings; and the potential for three groups/scenarios (Envoy, Mirkwood scouts, and orcs/easterlings).
Of course, the Elves get to Lorien ahead of the baddies and raise the alarm succesfully! Here's a thought -- perhaps they could do some 'spying' or something on the way and give some precious intel to Celeborn and Galadriel: something that really makes a difference in the battle?? (Could an Elf be successfully disguised as an Easterling. . .? A huge stretch, I know, but it would make for some interesting plot lines -- maybe not that big a stretch really: Frodo and Sam were able to pass for orcs!)