Does that still make author X an 'underappreciated author'?
No, I wouldn't say that, just a bit invisible on LJ, perhaps. (But would author X feel underappreciated? Somehow I don't think so, but I have no way of knowing that)
I imagined you'd simply have to do the incredible boring and time-wasting thing and check and double-check on the authors, before reccing them as an "unkown" or "underappreciated" author.

I know that if I was going to rec someone that I thought was unknown/underappreciated, I'd do that. But perhaps it's just me being weird :-)