The 4th Hussars Do Not Ride Donkeys!
The use of Caesar's Legion Command ability is not entirely clear to me, especially as it pertains to a Momentum driven activation. Let's say, for example, that 4 Legates pass in an effort to be later activated by a Legion Command during Caesar's activation. After that initial activation, if Caesar is able to pass a Momentum test, and therefore gain another activation phase, would Caesar be able to use his Legion Command again on those same 4 Legates and their Legion?
No. Caesar can only issue Legion Commands to Legates that have not been activated yet that turn (See 4.23 #5). If the Legate was activated during Caesar's first order phase, then the Legate cannot be activated again with a Legion Command during a momentum generated orders phase.
If you want to try and gain momentum with one of your Legates, then don't pass with him. The advantage of passing and allowing Caesar to activate your Legates is that it allows coordinated action by multiple Legions at the same time.
If Ceasar gains momentum, but all his Legates have been activated (either in the previous phase or earlier in the turn) then he is pretty much restricted to issuing individual orders or Line Commands to Cav/Aux units.