I was able to delay budbreak on a particular grape in my garden by emptying the ice tray a few times onto the soil around the base of the plant and then covering the ice cubes with leaves/soil. It delayed budbreak a few days, and will not generate enough cold to harm the plant. Don't know that it would really help you, but should not hinder. The cold moves down into the soil and draws heat from the soil as the ice melts.
Of course, if you did it too often you can create another problem of excess moisture, and in a container that is not too good.
Do you have excellent drainage in your containers or are they high in peat moss / organic matter?

sunniebel,
Please do not go out and buy yourself any of those
expensive artificial-snow-makers yet. Do a search in
this forum using the key word 'dormancy'. Many people
have had a similar fig problem before.
You should come up with some practical info/answers...

Sunnibel7
im in a colder zone but have similar circumstance with mine stored in attached garage.
They break dormacy earlier because when it just starts to get warmer out the garage temps get much warmer and wake up my trees. Worry about both day and night temps, i bring mine out when the temp is at least 50 out, then at night they get wheeled back in garage its just to cold and windy in early spring. When night temps start to match or exceed the 50 degree mark then i leave them outdoors. Mine are elder trees.
Yours as stated seem very young maybe a season or 2 old?
My suggestion to you would be leave it by window and dont try to make it stay dormant or slow it down its a young plant and there usually pretty vibrant at that age. When the weather starts to get warm bring it outdoors for a while then back in at night, do this for a while till the night temps do not drop so drastically, little by little is best. Perhaps a 60 degree minimum daytime temp is good to start with then back indoors at night, i bring mine out like i said at 50 but there older .
You said this
Also, I boldly root trimmed my Negronne after reading much info. here on the subject! (Scary!)I read that the tree should now be "secured to the pot" to allow the fine roots to grow without breakage...

Ok, how old is your negronne?
Is it in its final pot size,
I ask this as one usually does not root prune but simply pots up plant into bigger pot then when plant reaches final pot size they worry about root pruning in that pot, well either case after root prunning when you repot the plant it should not need support, only time i use support is for a smaller plants trunk to grow straight such as something like a wooden dowel or similar.
I hope this helps you some.
Best Luck
Martin

Hello! Thanks for all of you answering so quick!

Gorgi- I'm sorry, perhaps I should have mentioned that I already did a search on here about dormancy breaking early, but need clarification! What I read was about putting the figs out "when temperatures allow" and frankly I'm not sure exactly what temperature that is. However, Martin's post gives me a better idea of that.

Martin/Diesler- Thanks, that does give me a clearer idea of what I will need to do to keep my "baby" thriving! As for the Negronne, it was a 2 year old (from a cutting) tree when I bought it about a year ago from Edible Landscaping and the very nice lady there suggested I might want to do a root pruning this year. It is not in its final pot yet, but didn't really thrive last year when my other two did. Actually, the main reason these are in pots at all is because we hope to move in the next year or two and I want to take them with me! I got the bulk of my info. on the root pruning process from reading posts (and looking at pictures) on the subject by Tapla (Al?) and... James? I think it was. Anyhow one of them posted a picture of root-pruned, similarly aged tree (or perhaps younger)that they had aggressively pruned and that's where I got the stuff about securing the little tree afterwards.

And last (but not least), Boizeau, thanks for the idea! My soil mix, alas is a commercial potting mix. It does posess large pine bark chunks, but probably a fairly high ratio of peat as well. I would love to try mixing up my own mix but a lack of access to materials plus a pretty small number of pots makes it unlikely to happen soon!

Thank you all for lending me aid! I learn something new every day on these forums!

Sunni