> 1st of all...
> There is no free 7zip compression library, yet.

No, it exists:

http://www.7-zip.org/sdk.html

> 3rd of all...
> you may gain some bytes using solid archives...but on the other hand it nearly
> makes it impossible to remove/replace files...and decompression single files can
> be very nasty too

Solid compression is not so needfull... it's good if you compress very much files (as a source code)

> and last but not least:
> it's not supported by mame ;)

Well... I merge a support for 7-Zip archives for Mame (it's NOT the support of The Wndering Oddity). If you are interested, this is the link:

http://web.tiscali.it/u_fleku/