> In the context of picking battles, martyrdom is (self) sacrifice, with no direct gain for the cause.

On the first clause, exactly. (The second clause appears to be a bit of ad lib.)

> Abstaining from meat, to me, will take away one of the not-so-many joys I have in my life

I don't think the concept of 'martyrdom' encompasses self-interest. It does however consider the cause/s of other beings. So I maintain, not a very cognitively consonant use of the term.