I know that John - usually - does no beta versions.
BUT this one may be worth an exception since
a) 7-Zip v.4.42 is quite old meanwhile and
b) the actual 4.51 beta is not a "real" beta but an advanced version close to final (it runs veeery stable like all its predecessors and does - besides some new functions - contain lots of bugfixes over v.4.42)
Maybe worth the hassle of some pre-release Beta testing since it should be easy to update...
It's quite a bit of extra futzing for beta versions (is the bug in the portable packaging or the app itself?). I only usually do the Mozilla apps to make it easier for testers. Buy you can copy the beta of 7-Zip into 7-Zip Portable and it should just work.
I'll be packaging the next release of 7-Zip once it goes gold.
Sometimes, the impossible can become possible, if you're awesome!