I'm going to second that. If past years are any indication, I'll be the primary person doing work on the GSoC application. So from my point of view, that work is already laid out for me. I don't really have the time to work on that and Rails Girls SOC.
However, if you want to champion it, submit a proposal, and commit to mentoring, then I am in support of it. We should probably have at least two people willing to mentor, and I wouldn't recommend accepting more than one team. My personal priority will be mentoring GSoC, since I will be org admin if we are accepted, and suborg admin if we aren't accepted and work under the PSF/NumFOCUS). I suppose for SymEngine work you should get Ondrej's approval as well.
Finally, I want to point out that if your goal is to have more female contributors, this can be achieved under the auspices of GSoC as well. What would need to happen is for us to make a conscious effort to attract more proposals from women. I recall hearing from other mentoring organizations at the mentor summit who have done this and been successful.