Paul, your thread has gone unanswered for some time.
I Googled Aralias, mini, dwarf, but the closest answer, related to your question was growing Ming Alalias in small/bonsai pots.
Depending on your terrarium size and shape, you can place a Ming in a small container, then set, pot and all, under bottom material.

It doesn't mean there's no such thing as a mini Aralia, but not only did I Google, I also went through several plant books, came up empty-handed.

Mings are slow-growing..if you started with a small specimen, it'd take a couple/few years before it needed a larger container.
What size is your terrarium? Height?

Check out blackjungle for terrarium plants; they have some true beauty's. In fact, they specialize in terrarium plants. Toni

Thanks, Toni! I had noticed no one had any epiphanies so figured that there likely is no such beast. Pity.

The terr in question is a 55 gal. While I had read that mings are slow growing, I have had experiences with other "slow growers" that when put in a terr grew like they were on steroids (had an AV that grew from a 5inch diameter to be a foot in diameter in just a couple years before I finally ripped it out and rehomed it). Wanted to avoid the disappointment of having another plant go on an undesireable wild growth spurt.