Guesses only: Yes, it is possible to 'go back' to the 30p but depending on how the interlacing and de-interlacing is done, it may or may not be optimal.
One suggestion is that, due to AVCHD format officially supporting only 1080p24, 1080i50 [for PAL?] and 1080i60 [for NTSC?], Sony has to wrap its 1080p30 sensor output, and does by simply putting the "even" and "odd" lines of each "30p" frame in successive "60i" output frames. (But
Q1. Does this make sense?
Q2. Does this make it trivial to reconstruct the 30p from the 60i? (It seems so to me, but ...)
Q3. What about the 50i mode on models sold in "PAL countries"
P. S. The GH1 produces 1080p24 "native" and then wraps it as 50i or 60i, so maybe experience with that camera is a hint.
The bottom line seems to me to be that the AVCHD standard should have 1808p25 and 1080p30 added to the supported output formats! Then again, Panasonic disregards the restrictions by offering 1080p30 and 1080p25 in its AVCCAM HD camcorder, the AG-AF100, even though that camera's native format is 24p.
AFAIK, Sony and Panasonic are the authors of the AVCHD spec's, so this liberalization should be easy enough for Sony to arrange.