Okay, so I'm basically talking to myself here. Since 14 folks read the post, I figure 14 folks were curious.
Here's how it turned out:
I ended up painting the inside of the margarita glass with the perfect color glass paint (Cricket from Delta's PermEnamel) and then spray polyurethaning over the inside so it wouldn't melt when the water touched it. Coated the edge of the glass with tacky glue and rolled it in clear sparkly glitter. Cut a circle of green Oasis slightly smaller (about 1/2 inch all around) than the inside of the glass. Put it in the middle and filled the rest with shredded styrofoam, so that it'd look like chips of ice in case the paint defied me and started to run. Then filled with spider mums (they didn't have daisies or button mums in the right size) and roses, topped it off with two straws, and voila. It was a big hit, even if it didn't quite turn out like the inspiration.