Installing documentation with gems seems to be broken on master-1.9. Output below, confirmed on half a dozen gems. Installing with --no-rdoc --no-ri is a reasonable workaround for now.
I haven't investigated yet, just reporting.
Stevens-MacBook-Air:invoicing srbaker$ maglev-gem install isolate
[INFO] The MagLev team publishes customized versions of some gems.
These fix issues with the original versions or include optimizations
specific to MagLev. To allow MagLev to pick the originals anyway,
set the MAGLEV_GEMS_ALLOW_ALL environment variable to true
Successfully installed isolate-3.2.2
1 gem installed
Installing ri documentation for isolate-3.2.2...
ERROR: While generating documentation for isolate-3.2.2
... MESSAGE: a ArgumentTypeError occurred (error 2094), expected a Integer
... RDOC args: --ri --op /Users/srbaker/Development/maglev/lib/maglev/gems/1.9/doc/isolate-3.2.2/ri --main README.rdoc lib Manifest.txt CHANGELOG.rdoc README.rdoc --title isolate-3.2.2 Documentation --quiet
a RubySystemExit occurred (error 2752)
topaz 1> exit