Erm - not really, not by any dictionary I've seen.
(If you mean that the "target" spells must be of Concentration Duration - I don't think so. Any Duration greater than Momentary - and perhaps even that, altho' I myself wouldn't let that one go by. The spell is "sustained" - doesn't matter how long it was going to last, as long as it was going to last some duration longer than "snap".)
The word "sustain" is used in the ReVi Guidelines, and (more or less) means to "keep from failing/lapsing". The canon example is Maintain the Demanding Spell - which does use Concentration - but that's only one possible approach.
Btr, a spell w/ Target:Circle "affects everything within a ring drawn by the magus at the time of casting" - so no limit on the number of Targets involved - and with this ReVi effect, Targets = "spells".
So the only question is whether the spell(s) to be sustained are "within the ring" or not. I would say that, to meet this requirement with as few questions as possible, spells must meet all of: 1) must have been cast there, not from outside the ring/circle, b) must have a Range that does not extend beyond the circle/ring, and iii) must not create an effect which otherwise moves outside the ring/circle. (If only some of these are met, it might still work - ysmv etc. - but if you can meet all 3, it's less arguable - by the rules as written.)
(Note that, with some ring/circle effects, if anything relevant leaves the circle the circle is "broken" - in others, the circle's effect merely ends for that one traveling target.)
So, looks kosher btr. However, my own "cheese-dar" alarms are ringing loud in my gut, and I don't think many SG's would be happy where this particular effect might lead...
(Note also that 4th ed Guidelines are not nearly so tightly defined as 5th ed - really not useful except for inspiration, to then be re-built using pure 5th ed Guidelines and definitions.)