Planning a 4xD and 1xR AU? Help!

[wingzerosnuggles]

All right, so my sister requested I start an AU fic for her birthday this year.
She and I are both local history buffs and in love with the short story "May Day Eve" by Nick Joaquin. Basically the plot premise revolves around superstition, a "magic" mirror, young love and a marriage gone awry. Manila during the Spanish occupation was quite the colorful place with people of different ancestries running about the city. To name a few, there were Chinese traders, Japanese Christians on the run from prosecution in their homeland, Americans, and, of course the Spanish. You can see how the GW cast would fit right in while keeping history straight. Catalonia is Spanish, isn't it? And I could explain Quatre's ancestry as sort of a "mutt." Spain was occupied by the Moors for quite awhile so Raberba could have come from that. As for Winner, I could use the American aspect. Heero and Relena would fit right in with the Japanese and the Americans.

Now here lies my problem...I've never written AU before. Any tips? I studied my material well enough. It's been drilled into me since elementary school and I've read "May Day Eve" too many times to count. Also, watched the play... I'm just not exactly sure I can pull it off. I don't really trust myself with serious multi-chapter either though I doubt sis will let me live unless I finish it. :p Also, I don't take my dose of college Philippine history until next semester so I'll be drawing from my high school background. Actually, that's not too bad a base considering it was a requirement to wear the fancy aristocratic clothes from that era every Monday of August for my four years in that school as part of long-standing tradition(Damn itchy fancy dress made from pineapple fiber -_-). I
have method experience in that respect.

Basically, I'm asking if you think it would be wise to proceed with this now? Is it a good enough concept? Is it plausible? Would it hold interest? I'm actually torn between writing this purely for my sister's eyes and posting it...