"My first question is, just exactly how visible is discussion about gay/lesbian and related issues within the Muslim community, especially in the U.S. and Canada?"
To be honest, I couldn't really say. While homosexual behavior for either sex is considered a major sin in Islam and is condemned as such, I do know that there are gay Muslims and some people who support them (almost exclusively in the "progressive" Muslim camp). However, the vast majority of Muslims in North America (let alone around the world) do not agree with the progressives on this topic.
"The only lesbian Muslim voice I can think of off the top of my head is Irshad Manji, but I have no idea how she's been received, or how representative she is."
Irshad is widely condemned among orthodox Muslims, nor is she representative of us. At this point in time, I couldn't even say if she still considers herself to be a Muslim, although that's not for me to decide. (Allah (swt) will judge her concerning that matter.) The fact that Irshad is a lesbian is almost beside the point; she is a pariah to most Muslims because of her unIslamic thoughts and beliefs. The problem with Irshad, from our perspective, is that she tells you (the non-Muslim community) only what you want to hear; she doesn't say what orthodox Muslims actually think. In that respect, she and others like her cater to non-Muslim prejudices against Islam and Muslims.
"My second question might require the perspective of a gay or lesbian Muslim, but it would seem to me that salat, as you describe it here, would pose a special challenge for gay and lesbian worshippers. If there is open discussion about the issue in worshipping communities, does salat pose one of the challenges to dialogue on the subject?"
You're right in that I'm not really the person to ask; however, I will say that how we pray (with the sexes segregated) isn't going to change in 99.99999% of the masjids worldwide (or even in North America). Other than that, I really can't say.