This is an interesting 11-year old discussion for me... Having just changed strings about 12 days ago, my 2nd complete string change in 2 to 3 months, I'm reminded how nice it is to be able to bow again; the previous strings that I was experimenting with were just not designed for arco, and they really weren't bow-able at all. I'm glad to be able to bow again with my current strings.
But in reading this thread, this discussion seems to be much more than about the ability for a string set to be bowed; it's seems to really be about the tone of a set of strings under the bow, while the ability to successfully bow that set is pretty much assumed. And the tone under the bow seems in my simplistic view to be a couple of magnitudes more complex than just being able to start the strings with the bow.
That said, for anyone wondering, it is very possible to start some fat, low tension synthetic strings in arco if those strings have some surface texture to them either by virtue of windings or as a result of a special process. Most makers of fat low tension synthetic strings don't intentionally target their strings for arco, but for many brands it can "accidentally" be done at least to some extent. There may be some left-hand performance issues because most of these fat low tension synthetic strings will require higher action than thinner high tension metal strings, but despite being challenging, at least "simple" arco is doable with a number of brands of these strings.
In particular, for me probably the biggest arco surprise among fat low tension synthetic strings may be the diminutive Superior BassWorks (SBW) Deluxe "Dirty Guts". These are fairly popular among slappers and pizz players of various roots genre; they are very low cost, basically Kevlar core gator line that has been textured with a spiral cut and that has had provisions made for reliable attachment to double bass tailpieces. They come in a bump-pack of 6 strings so people can select their strings by thickness and tension, although they are by nature all pretty low tension strings. Because of their texture and spiral cut, they have an amazing affinity for the bow, probably better than most other fat low tension synthetics... And in fact they seem to sound pretty good for arco (although again, arco tone is probably not something I can judge very well).
I've also had fairly good luck with Innovation SilverSlaps for arco. They aren't as responsive as "Dirty Guts"; fresh sticky rosin, a tight bow and some special technique may be required. But they seem to be reliably started under the bow with a little practice. These strings growl in pizz, so they also growl in arco, but they can produce a sensitive quality in their tone too.
Respectfully, most arco specialists who really know excellent arco strings may dispute the value of being able to start these fat low tension synthetic strings with a bow, but to many of us seemingly "permanent arco novices" who may only bow during practice or just for rare effects on stage, this level of arco ability may still be of high value. I know that I really enjoy the limited bowing that I do, enough that in the future I may require at least moderate bow-ability in the fat low tension synthetic strings that I install.