Cleon
Legend
I like your proposal for wormwork. Let's go with that.
Guess that spell's done then. Updating Wormwork working draft.
A regular purple worm is CR 12. So we just need something to make it extraplanar to be summoned without changing its CR (much), I guess. If we don't have a suitable template in the CC, we could probably specify it summons one from the ethereal plane or something. Or we could look for an appropriate monster in the CC.
Had a look through the Creature Catalog and couldn't find any templates that would add the Extraplanar subtype to a Magical Beast.
We must have something wormy and extraplanar in there.
Well there's the "Wraithworm" which is a Magical Beast (Extraplanar) but is also a snake with energy drain AND venom but is CR 5 and can be turned/controlled as if it were an Undead. Doesn't feel appropriate for a generic worm-themed summon monster IX.
The Tenebrous Worm is an Outsider that's basically a giant caterpillar not an earthworm-type creature. Plus the conversion's 3.0 and I'd rather use a 3.5 one.
There's a few worm-ey creatures with the Earth subtype that don't have Extraplanar, like the Glitterworm. I thought we had a conversion of the Thoqqua in the Creature Catalog too, but can't find it. That's a Fire creature though, so would seem unsuitable despite its resemblance to an earthworm.
Overall, it would be easier just biting the bullet and add an arbitrary note that the summon monster IX spell conjures a Purple Worm with the Extraplanar subtype without bothering with a template.