I"ll qualify my last statement by admitting that I am...by FAR...the most active GM in my league. So I usually have a pretty good idea who the untouchables are and who's likely to be moved on any given roster. If you put the time in...you'll know more...and the more you know...the more likely you are to be able to pull off these unlikely deals.
(Trading for Waiver picks...when waivers are limited is an interesting tactic that I've mastered as well). But I'll save that for another day.