I'm U152/L2/Z49/Z142+ L562-/L594- and I'd like to test for Z150 downstream. Geno 2.0 does test for this SNP, but instead of forking over more money to get the entire gamut of non-U152 SNPs tested for by Geno 2.0 now, if possible I'd rather save some money by waiting until FTDNA offers an a la carte test for Z150 at some point down the line.
What are the chances that I could be waiting indefinitely for a particular a la carte test to become available?
These chances are significant, as FT-DNA are offering an à la carte test for only a minority of all known SNPs and a few have been "in the pipeline" for a significant amount of time.
In your case it might be an idea to get a little bit more active and prepare a full business case for FT-DNA; after all they are in this trying to make money.
Get a group of people together who confirm in writing (email) that they are planning to order the SNP when it gets available. If you have such a list (and I guess one or two people will be less than impressive), we will suggest further steps.