As the authors rightly state, the /r/ pops up mostly after word-final /ɑː ɔː ə/ and after diphthongs ending in schwa. The majority of cases involves the vowels /ɔː, ə/, less often is the /r/ heard after words ending in /ɑː/.
Our reliable phonetic phenomena spotter and rapporteur Paul has recently come up with two examples of unetymological r-liaison after a PALM vowel (i.e. /ɑː/):
1. "[...]to do the cha cha ch /r/ isn't it."
2."It's a 7-foot grand piano made by Yamaha/r/ * uhm It's'n amazing piano."
This second snippet seems to be less straightforward because I hear the /r/ and next a /z/ followed by a very brief hesitation sound and then, finally, the "it's'n" is uttered.