R̥ and r̥ (and R̥̄ and r̥̄ and L̥ and l̥ and L̥̄ and l̥̄) can be made using “combining diacritical marks”. The explanation is in the Brill Typeface User Guide p. 2, in the section “How to use combining diacritics”. [ṚṛṜṝḶḷḸḹ are, of course, also available as precomposed characters.]
There is one remark in in the Brill Typeface User Guide I must modify for the current release of the Brill fonts: contrary to what I wrote in the manual, the combining mark(s) above should for the time being be keyed before any combining mark(s) below. (I wrote: “When a base character carries one diacritic above and one below, these two can be added in either order”.) I just found that out when keying the long variants of vocalic liquida, above.