The published feature description suggests that this could be done with either GSUB or GPOS. I don't recommend doing it with GPOS, though, because that will create issues where glyphs are kerned; these issues could be resolved with further, contextual GPOS adjustments, but that's a lot of tricky work, whereas GSUB makes everything much simpler.

I am not a volt expert(although I use Volt form time to time), but why do ths in GPOS att all???

Thanks John, I hadn't considered the kerning implications, in that respect GSUB is the more logical choice.

@eli This feature is not unique to VOLT built fonts and there are many fonts out there that have used GPOS for this feature. In FontLab GPOS can be created easily in a single line of feature code, but as John pointed out, this may well throw up some additional issues.

Malcolm: This feature is not unique to VOLT built fonts and there are many fonts out there that have used GPOS for this feature.

Are there? I'm having trouble thinking of any.

Perhaps I'm wrong John but I didn't Adobe first try this feature with GPOS then switch to GSUB? There is probably a few of these still around.