Good news! Turns out the issue here was due to an attribute in the Create profile that was enabled as a Static value but had a null value. This is apparently not allowed.
Once I populated a dummy value in there, I was able to provision with no issues using the regular entitlements, with the Add Entitlement operation being a copy of the Create Account operation. Even though there is no entitlement actually added, this seems to not be a problem and IdentityNow is perfectly fine with it.
Not exactly the same issue, but shout out to @zbetz and this post for pointing me down the right path!