I'll preface my answer by saying I'm not a weightlifting coach, and if you don't have one I highly recommend finding one while you work out your form issues. That said, as someone who trains for strength, there's some pointed questions I have to ask.
- Are you sure it's a biceps inflexibility and not a triceps weakness? If you contract the triceps you will be stretching the biceps, and vice versa.
- Are you sure it's not a shoulder inflexibility keeping the bar forward of an optimal balance position for you?
I understand hang/power snatches are very different animals from true Snatch (with full squat). However, the issues I've run into with hang snatches that are similar to the issues you seem to be dealing with have to do with shoulder flexibility or triceps weakness that had to be corrected. If your shoulder is tight and you can't get the bar where it needs to be, you almost have to bend your elbows to compensate for the balance problems that causes.
An exercise that can help with both issues is a behind the neck snatch grip press, or sometimes known as a Klokov press. You may have to start with just a PVC pipe while getting used to the motion--same as some coaches would start you off with as you learn the motion (such as Mike Burgener).
Another exercise that can help with both issues would be a snatch balance. It's similar to the Klokov press, but is much more explosive.
An article by Dan Pope about Mike Burgener's warmup routine looks like another good resource. For Olympic weightlifting advice, both Mike Burgener and Glen Pendlay are some of my go to resources. You'll also see that the article references the same Klokov video I linked to above.
There is one more video I wish I could link to, but due to copyright issues I can only tell you how to get it. There's a book called 365 Strong by Brandon Lilly that you can purchase at the Juggernaut Training store. Brandon Lilly is a power lifter and strong man, but in the book is a link to a video you can download and watch on how to "Mobilize like a Weightlifter". The instructors are part of the Juggernaut team, but there's some good stretches that deal with both the shoulder and the arm flexibility at the same time. The one that I'm thinking of that sounds like it would apply best for you is described below:
- Get a strength band and attach it to a power rack (or pull up bar)
- Grab the band with your arm facing behind you and kneel down on one knee, keeping the shoulder in a neutral position (i.e. don't let the band pull the shoulder to the top)
- Hold for time, and then switch arm and knee. (NOTE: the knee that is up should be on the same side as the side you are stretching
That particular stretch helps stretch out the lats, delts, and biceps. There are other good mobility tips in that video, but this one should help a good amount.