This basically is also done by he GPO, you could edit %windir%\inf\system.adm to include the drives you want to hide or prevent access to.

Code:

		POLICY !!NoDrives
			#if version >= 4
			SUPPORTED !!SUPPORTED_Win2k
			#endif

			EXPLAIN !!NoDrives_Help
			PART !!NoDrivesDropdown	DROPDOWNLIST NOSORT REQUIRED
				VALUENAME "NoDrives"
				ITEMLIST
					NAME !!ABOnly           VALUE NUMERIC	3
					NAME !!COnly            VALUE NUMERIC	4
					NAME !!DOnly            VALUE NUMERIC 	8
					NAME !!ABConly          VALUE NUMERIC 	7
					NAME !!ABCDOnly         VALUE NUMERIC	15
					NAME !!ALLDrives        VALUE NUMERIC	67108863 DEFAULT
					; low 26 bits on (1 bit per drive)
					NAME !!RestNoDrives     VALUE NUMERIC	0
				END ITEMLIST
			END PART
		END POLICY

You can add as many drives as you want using the same hex codes. ABC would be 1 + 2 + 4 = 7
Keep in mind that every line you add to that that you also have to specify that in the [strings] section of the system.adm file.