There just aren't that many adjectives that end in unstressed -ey and are commonly used in the comparative or superlative, at least in formal writing.
OneLook Dictionary Search turns up 49 adjectives ending in -ey; with false positives removed, I would give the resulting list as follows:
boney, cagey, choosey, clayey, cockney, curvey, dicey, dickey, dopey, flakey, flukey, gamey, gluey, gooey, goosey, grapey, hokey, holey, homey, mangey, matey, motley, mousey, multistorey, nosey, phoney, plaguey, pokey, pricey, ropey, scaley, scarey, slatey, spacey, stagey, tiddley, whiney, winey.
Taking a look at some of the more promising of these, there are sometimes hits on Google Books for forms such as "diceyer" (in quotation marks) or "priceyest", but I haven't seen any data that would make me conclude that there is an established rule worth worrying about.
I wouldn't really call -ier or -iest in this context a "pronunciation spelling", though, just a variant, as often exists even in the case of the positive endings -ey and -y.