You could do it last August from Llandudno to Porthmadog (FR) but not sure if you could go any further.
I tend to agree. As far as I can ascertain, you can book through fares to or from the FR (to Minffordd/Porthmadog FR southbound or Blaenau F FR northbound) but not
via the line between National Rail stations.
LLANDUDNO (2484) to MINFF/PTHMDG FR (H074) anytime day single £19.70, anytime day return £24.40.
Porthmadog to Criccieth is £2.40 anytime day single, £2.50 anytime day return.
Day rovers for N Wales do not include free FR travel (only the all-zones £25 giving a 50 per cent discount) and the Round Robin (£29) would be too expensive just for this part of the circuit.
I agree with 317666, though, that for the full circuit the Round Robin is quite good value. One thing that puzzled me whe I travelled on one, though, was that the outward half wasn't collected/clipped/stamped on the FR, as it's only supposed to be for a single journey. Unmarked, there'd be no way of knowing if anyone used it to return, as the way the origin and destination are presented, the person could claim to be doing the circuit the other way round. I must stress that I did not misuse it, and would in no way condone ripping off a heritage line, but the FR ought to consider the possibility.