Just a quick update, a friend on Blu-Ray.com translated the specs for the upcoming Toho Blu-Rays from this site:
www.toho-a-park.com

The specs for Mothra are:
Features:
1. "Brand New Mothra" (1974 Champion Festival Release Edit / HD Blow-Up)
2. "Mothra Document" (tentative) (HD quality) [NEW FEATURE!]
3. Audio Commentary (Hiroshi Koizumi)

Audio:
1. Mono (Linear PCM)
2. 4ch Stereo (Linear PCM)
3. Audio Commentary (Dolby Digital)

Subtitles:
1. Japanese