From the Macau Tourism Website:
e) The "Hong Kong Identity Card" (with symbol “*”, “***” or “R”), "Hong Kong Permanent Identity Card" or "Hong Kong Reentry Permit" holders may stay in Macau for a maximum period of one year.
So that means that the following non-permanent HKID holders can enter Macau on the same basis as HKPIC holders.
1) Chinese nationals of the mainland who have been granted unconditional stay in Hong Kong and eligible for a Hong Kong re-entry permit.
***UX or *UX ( Or with a Hong Kong re-entry permit if they're under the age of 11)
2) Stateless people who have been granted unconditional stay in Hong Kong and eligible for a Hong Kong re-entry permit.
***UO or *UO (Or with a Hong Kong re-entry permit if they're under the age of 11)
3) People with the right to land in Hong Kong.
RO or RZ
I never had problems using my "R" HKID to enter Macau. There has been a few times where the immigration officer was not sure if they could stamp my arrival card with a non-permanent HKID. They'll usually just ask their supervisor or their co-worker next to them if "R" is ok. Once they get a "yes" , they'll stamp my arrival card. This happened to me last week at the Hong Kong Macau Ferry Terminal.