Sure, you can offer, but I'd bet that they'd refuse it, even on repeated offers, especially if you'd gotten a complimentary upgrade. If I were a front desk CM, and I knew there was a possibility that I could be let go for taking money to give extra perks to a customer, I'd probably be declining all tips, just to be safe.
PP's have talked about the culture at Vegas vs. Disney, and that plays in, too. Tipping the front desk clerk is almost part of the theming in Vegas. It's got a little of that outlaw feel to it.
All hotels set their own policies regarding complimentary upgrades. Disney has a fairly rigid set of rules around when they can be offered or given, and they're not given very often at alll, when you consider that thousands of people check into Disney resorts every day. Most are assigned before you even arrive at the resort; less commonly, a CM will have an upgrade to give out at their discretion, and even then, they must document and justify it.
I liked the 'negative pixie dust' comment, because it kind of boils down to this:
Vegas clerk getting offered 20 bucks for a better room: woohoo, an extra 20 bucks in my pocket, I like this guy.
Disney CM getting offered 20 bucks for a better room: oh, great, this guy wants me to do something that could cost me my job.
Entirely different vibe.