Instrument rated pilot (Joe), without complex endorsement, acts as a
safety pilot for another instrumented rated pilot (Bob), in Bob's
complex airplane.

Reg's regarding "general" safety piloting state that both can log PIC
time, given an agreement is made that the safety pilot is responsible
for the safety of the flight, although conscience may dictate
otherwise. However, acting as PIC means:
1.Has the final authority and responsibility for the operation and
safety of the flight;
2.Has been designated as pilot-in-command before or during the flight;
and
3.Holds the appropriate category, class, and type rating, if
appropriate, for the conduct of the flight

So... Category and class rule out Joe logging PIC time, but how does
that affect the legality of acting as a safety pilot? With Bob under
the hood, Joe's participation is a requirement...

Safety Pilot is Required, Qualifications of (FAR §91.109)
FAR §91.109 says a safety pilot is required for simulated instrument
flight (as a required crew member) and that the safety pilot must have:

a valid private pilot certificate §91.109(b)
appropriate category & class ratings §91.109(b)
a current medical certificate §61.31(d)(1); § 61.51(e)(1)(iii),
§ 61.51(f)(2), § 61.3(c); §
61.56 (c), § 61.57(c), & § 61.3(c)
and meet all the other pilot-in-command currency requirements
§61.3.

"appropriate category & class ratings §91.109(b)" indicates to me
that Joe cannot legally be "the eyes" for Bob, because Joe does not
have a complex endorsement...

Correct?

Best Regards,
Todd