Chatted with a frequent flyer friend who says he often does the following, flying J class.

1. Find the right J class outbound
2. Book it online with a random Y class return, open jaw if needed e.g. to YUL if going to YYZ. You won't need this, so doesn't matter really what it is as long as it generates the same point reduction as your eventual desired ticket. (Since your outbound is J, you will get deducted the J level of points even if some of your flights are in Y)
3. When J class opens up on your desired connections and return, phone in to make the change. Pay the $90+tax change fee, plus minus the change in fees and taxes.

Two caveats -
You're saying you'll be booking the outbound as soon as loaded into the system; so I'm not sure what restrictions (e.g. only some airlines that load availability earlier?) you'll have for the fake initial return.
Reiterating that a paid economy ATH-PRG will likely be available very cheaply -- comparable or less to even one change fee on an aeroplan reward. So the manoeuvre may not be worth it.