Not at all, this is your wish it is understandable, thanks for sharing either way!:)

since Standard is missing the Expansion Pass DLC.

The Hitman model of how they're selling the games and levels is interesting, but I have to think there must be a less convoluted way of pulling it off.

Interesting, sure. I mean, technically, Standard includes the first expansion pass (of two), but given that Hitman only has a couple of episodes without the passes, it'd probably be more natural to consider than as "Base Content Pass" and "Expansion Pass", respectively. So yeah, as you said, it could be less convoluted- or at least, less misleading. :P

Heck, even the listing for Gold seems confused about it:

Buy HITMAN 2 - Gold Edition
Includes: Expansions 1 & 2, and exclusive in-game items.

HITMAN 2 - Gold Edition is the ultimate version of the game and includes all available content from the Standard Edition and the Expansion Pass.

:P

so would it be considered in poor taste to add the requirement that people don't already have the base (paid version) of the game now that it's already well underway?

This is one of those circumstances where, natively, both sides can be considered to have some level of expectations and obligations, rather than anything falling primarily just to one side.

Thereby, it's only in poor taste in circumstances where there's a pre-existing expectation that the GA creator wouldn't reroll. For example, if a base game had been previously free or low-tier-bundled, then entrants can have an expectation that if nothing is mentioned, the GA creator is fine with them entering for that game's DLCs alone.

Otherwise- that is to say, as with a circumstance like this- it'd be more natural for entrants to ask before entering (y'know, assuming they're entering properly, rather than by using scripts). It's definitely not something you'd need to feel bad about, at least. Nevermind that it's a public giveaway, which isn't exactly known for having users who read the descriptions to begin with. >.>