Please find a proposed patch that should fix this issue. I haven't written scheme in many years, so please excuse me if I have failed to follow convention in any way - I won't be offended at all if you want to refactor it. This patch should apply cleanly on the current git master branch and also on the v0.8.1 release.
This patch modifies http-fetch (in build/download.scm) such that it calls Guile's open-socket-for-uri after fixing up the uri so that it always has a port specified. I'm not sure how to test the bootstrapping NSS issue that required open-connection-for-uri, but my expectation based on reading the source is that this alternative should work for Guile > 2.0.7, and I've left the original open-connection-for-uri in there for backwards compatibility with Guile < 2.0.7. If someone can test this against a situation known to have needed the NSS workaround, that would be great.
I've also changed the only other call to open-connection-for-uri, which is in the probe-uri function in scripts/lint.scm - my suspicion is that won't be an issue because I'm guessing the lint scripts are not used while bootstrapping, so the open-socket-for-uri function will probably be fine for the purpose of probing whether a URL is valid.
After applying this patch, guix should use the http proxy support built in to Guile >= 2.0.10. This appears to be working for me - I've just started a bootstrap and it has now started to successfully download packages - so far, so good.
Please let me know if there is anything else I can do to assist with getting this issue fixed.