I have been to Edziza Park and surrounding area several times in late summer. Stunning country. Not sure exactly what your goat area is but, yes, Mowdadee Lake is an option. The other option is Buckley Lake but that it a long way to hike or by horseback into goat terrain. That's more for caribou and moose. If you are on the south edge of Edziza park you can fly into Arctic or Little Ball Lake (usually pilots don't pick people up from Little Ball because of the size of the lake). Both of those lakes are south outside of the park. If you head north you'll hit Spectrum Range, Artifact Ridge etc. which is prime, I would guess for goat. But that might be too far south anyway. I saw more sheep (ridges south of Cocoa & Coffee crater), grizzly (everywhere), wolf and caribou than goats. Admittedly, I was not hunting up there at the time...
Float plane out of Telegraph Creek or Tattogga Lake are your best options. Tattogga is closer. Telegraph adds another couple of hours at least.