The answer is in your question... they can't ! 
Which means they infringe possibly the most important ColReg's rule. 
Solo racing should be restricted to 24h.  

Any race starting in France must be declared to the maritime authorities who will impose conditions before authorising the race. Up to now they have tolerated single-handed sailing.
For the Mini Transat all boats are required to carry 2 trackers

There's nothing in IRPCAS Rule 5 about 'permanent'.  Here's the rule.

Rule 5
Look-out
Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision.

The requirement is that the lookout be 'proper', that is, sufficient according to the circumstances.

Whether lookout is proper or sufficient will only ever come into question when there is a collision or a failure to keep out of the way or otherwise manoeuvre as required by the rules, that is when vessels are within sight of or close to each other.

Clearly enough, when there are no other vessels about, it may be sufficient to keep no lookout at all, at least for some periods.

Use of radar and AIS alarms may assist in extending intervals between visual scanning as a lookout.

Solo sailors have always faced this challenging issue, mostly successfully due to their high level of skill and commitment to proper seamanship. Now days, AIS is a great asset. However, the recent collision of merchant ships in the North Sea reminds us that there are fully-crewed ships not being sailed in a seamanlike manner. So we sail the seas at our own risk. 

 In 2 weeks

Arkea Ultim Challenge-Brest, the first ever single-handed Ultim round the world race.

ARMEL LE CLÉAC'H  said: "....... and I won't hide from you the fact that sleeping at 35 knots of speed, or even 40, is a lot more complicated than on a monohull "