I believe that "available means appropriate..." allows the use of radar and AIS with alarms.

Yes, solo racing has always teetered on the edge of legality, especially before efficient onboard power generation, AIS, and marpa-tracking radar. Since Rule 5 requires "continuous, proper lookout by *all* means available," then you are clearly not complying with Rule 5 if you're sleeping, if you are down below eating, making repairs, etc. and there are places - like crossing shipping lanes or other high traffic areas where you really need to comply with the letter as well as the spirit of the rule.

AIS and Radar give some ability to monitor when eyes aren't "outside the boat," but many commercial fishing vessels don't use AIS and other types of vessels (including other sailboats - perhaps even the one you're racing on) provide poor radar returns. 

I think the reasonable compromise is that violation of Rule 5 only becomes an issue if a collision or near-miss occurs, and you can't prove you were maintaining watch under the requirements of Rule 5. Bear in mind that outside of land there's not really an enforcement mechanism - it's not like the IRPCS police show up off the continental shelf and charge you with sleeping. In the event of a collision, however, you can be sure the responsibility would be assigned accordingly.