I've included Colreg 30 below. As I read it, for a boat under 50 meters, you can put the anchor light wherever you think it can best be seen (although I have also worried if my lower light would count as ARW given that is blocked by the masts at certain angles). Further, part C suggests that even if you are using an ARW at the top of the mast, there is no reason you cannot add additional lights down low to help improve visibility.
Rule 30
Anchored Vessels and Vessels Aground
(a) A vessel at anchor shall exhibit where it can best be seen:
- (i) in the fore part, an all-round white light or one ball;
- (ii) at or near the stern and at a lower level than the light prescribed in subparagraph (i), an all-round white light.
(b) A vessel of less than 50 meters in length may exhibit an all-round white light where it can best be seen instead of the lights prescribed in paragraph (a) of this Rule.
(c) A vessel at anchor may, and a vessel of 100 meters and more in length shall, also use the available working or equivalent lights to illuminate her decks.
(d) A vessel aground shall exhibit the lights prescribed in paragraph (a) or (b) of this Rule and in addition, where they can best be seen;
- (i) two all-round red lights in a vertical line;
- (ii) three balls in a vertical line.
(e) A vessel of less than 7 meters in length, when at anchor not in or near a narrow channel, fairway or where other vessels normally navigate, shall not be required to exhibit the shape prescribed in paragraphs (a) and (b) of this Rule.
(f) A vessel of less than 12 meters in length, when aground, shall not be required to exhibit the lights or shapes prescribed in subparagraphs (d)(i) and (ii) of this Rule.