We know from the introduction of the Morgaine saga that there were ties to Union-Alliance.  I'm sure this is what CJC might have in mind.

Aja Jin wrote:

Pretty often, one sees posts online regarding unification of the Foreigner and the Alliance/Union books. This is often followed by "She said they are different universes", and discussions about difference in hyperdrive/jump technologies. This is all correct.

However, there is another "technology". Qhal gates, which exploit some unknown Physics that seems unrelated to jumps, but provide a way to move from place to place, and from time to time. 

So what if the gates also work "between" universes?  Why not? And Bingo, we have a solution that lies within the known bounds of her universes.   

Yes, that would be a possible solution.  If we had a few more clues about how the gates work ... ?  I can see a close re-read of the Morgaine books in my near future for just such research purposes.

Meanwhile, I am reminded of another possible connection rabbit-hole I haven't finished exploring yet. I think I'll prioritise that to this week's research task and see where it leads.
 

Here's the relevant snippet from the prologue to Gate of Ivrel that purports to be from a Union Science Bureau document:

Prologue (p 10 Orbit / Futura pb 1977)
The Gates exist. We can therefore assume that they exist in the future and in the past, but we cannot ascertain the extent of their existence until we use them.  According  to present qhal belief, which is without substantiation, world upon world has been disrupted: and upon such worlds elements are greatly muddled.  Among these anomalies may be survivals from our own  area, which might prove lethal to us if taken into backtime.
It is the Bureau’s opinion that the Gates, once passed, must be sealed from the far side of space and time, or we continually risk the possibility of another time-implosion as ruined the qhal.  It is theorized by the qhal themselves that this area of space has witnessed one prior time-implosion of undetermined magnitude, perhaps of a few years of span or of millennia, which was occasioned by the first Gate and receptor discovered by the qhal, to the ruin first of the unknown alien culture and subsequently of their own.  There is therefore a constant risk as long as there will ever exist a single Gate, that our own existence could be similarly affected upon any instant.  It is therefore the majority opinion of the Bureau that utilization of the Gates should be permitted, but only for the dispatch of a force to close them , or destroy them.  A team has been prepared.  Return for them will of course be impossible; and the length  of the mission will be of indeterminate duration, so that, on the one hand, it may result in the immediate entrapment or destruction of the team, or, on the other, it may prove to be a task of such temporal scope that one or a dozen generations of the expeditionary force may not be sufficient to reach the ultimate Gate.
 
Fyi.

Yes, while the later books all contain versions of the prologue that repeat and paraphrase the same information and background to the stories, Gate is the only one to explicitly cite the Bureau and imply Union as the source of the closure team.  The later book prologues do confirm the initial size of the team as 100 individuals.