I believe that was the general idea to do; I've wondered for some time what we were using for the actual colonize ability, and didn't realize it was vanilla until recently. We have, I believe, a separate model for the D20 Heron, those big dropships from the Spirit of Fire, we could definitely use them for the particle, though as a model it may put unnecessary stress on the game. That said, I think colonizing happens rare enough where it wouldn't pose a optimization issue, I mean we already use the phantoms, spirits, seraphs, pelicans, longswords, etc, for planetary invasion particles... So why not use the D20 Heron for colonization, make something for the Covenant, and implement?
Or, rather than the D20 Heron, as it's only known to be used by the Phoenix-class support vessels, use the D96-TCE Albatross - though, that would require a new asset be made.