AFAIK, to use the brightbox packages on heroku you'd have to write a buildpack that either downloads all the dependencies and runs through the same build process that brightbox uses, or use something like heroku-buildpack-fakesu to create a fakeroot type of environment where you can install debs.
Another issue with using the brightbox packages is that if you happen to run into a bug, you'll have to figure out if the bug was caused by something non-standard in your ruby installation or if it's actually a bug in ruby.
One last thing to note is that it looks like the latest brightbox release targets patchlevel 327, while ruby core is at 392 (not counting 2.0.0-p0), so you're missing a lot bug fixes until the brightbox team gets around to building against the latest release.
Another issue with using the brightbox packages is that if you happen to run into a bug, you'll have to figure out if the bug was caused by something non-standard in your ruby installation or if it's actually a bug in ruby.
One last thing to note is that it looks like the latest brightbox release targets patchlevel 327, while ruby core is at 392 (not counting 2.0.0-p0), so you're missing a lot bug fixes until the brightbox team gets around to building against the latest release.