That's not as silly as it sounds.
If a certain Domestic airline really wanted to stick it to a certain Regional airline, maybe for not playing nicely during some recent merger negotiations, then the ideal way of doing that would be to decimate the ranks (of said Regional airline) of it's ATPL holders in order to crew it's fledgeling Turbo-prop operation. One way of eliminating the opposition in one fell swoop! How's that for a conspiracy theory.
Blueskymine, if there is some sort of "agreement", trust me it'll evaporate quicker than an ice cube on a SAAB dash in summer once commercial necessity comes into play. Realistically however, you may find that while Jetstar has fed you the line about helping out REX, it probably has more to do with their crewing dilemas WRT the possible transfering QF pilots, and their own logistical problems. The REX line is just their way of saying they have no F#cking idea what's going on with their crewing at the moment! Of course they're too arrogant to admit that to you.