On the forum I find prince-opentype(onum) and prince-opentype(frac) and documentation tells me one can have multiple featurs in http://www.princexml.com/doc/properties/font-variant/
However, I do not see the list of what other OpenType features may be supported, e.g., All Small Caps. Is that list available?

Any OpenType substitution feature name may be specified, but small-caps can already be enabled with the built-in small-caps keyword:

font-variant: small-caps

This will use the OpenType "smcp" feature if it is available.

I need both old-style numbering and small caps, and would also want to have the option to use the proportional variant of the figures. Is there an exhausistive list of all prince-opentype features?

Ah then I guess you will need onum, pnum, and smcp. Microsoft has a list of the OpenType feature names here.

Great, thanks also for the link. On the topic of documentation: you may want to stipulate the separator between features; I noticed that prince-opentype(smcp,onum,pnum) works and prince-opentype(smcp onum pnum) does not.

Right, we will do this.