cessnapete
Can't add much to what p0tt3r has written, other than add that there's certainly no talk on the line of cruise only pay scales or long term cruise only contracts.
FWIW BA tried a similar temporary cruise only system briefly on the 744 a long long while back when most of it's work was ultra Long Haul which meant it was difficult for co-pilots to retain recency. It ended up being difficult to administer and meant scheduling lost flexibility with no financial gain to the company. It didn't last very long.