I wondering if it's possible to control the 24hr API request limit at the user level.
For example, if I have a 100k 24hr limit, I may want to prevent a specific integration user from consuming more than 50k of those.
Is this possible?
No. You cannot control API usage at User level, it's always aggregated at the Org level irrespective of a User.
You can find the reference for this on System Overview: API Usage documentation (emphasis mine):
Limits are enforced against the aggregate of all API calls made by the org in a 24 hour period. Limits are not on a per-user basis. When an org exceeds a limit, all users in the org can be temporarily blocked from making additional calls. Calls are blocked until usage for the preceding 24 hours drops below the limit.