It definitely does play DVD+R DL. I've tried DVD+R, DVD+RW and DVD+R DL (all by Verbatim), and they all played without a hitch.
Not sure about the chip used. The Sigma 8635 is a good guess, but I can't find any info to confirm, and I don't want to crack the case open until I'm pretty sure I won't need to return it.
I believe this is a rebadged and modded Sigmatek SBR-1000
. Same remote, similar menu design, similar front panel, identical back panel. And according to this post
, the Sigmatek itself is a Funai clone. So tracking down firm specs on the Momitsu is a bit a of challenge.
P.S. An insider from Sigma who is answering questions in this thread
on Blu-ray.com seems reluctant to answer the question about whether the Sigma chip is in the Sigmatek player.