I tried checking a few online dictionaries and can't get a feel for whether the word is generally used in a positive or negative sense.
What is the connotation of "dissimulation"?
Its hard to say, simply because it's not at all a popular word anymore. The meaning indicates negativity, in the same way "half-truth" generally does, but I've never encountered it in general speech or common use. I had to look it up to make sure I knew what it meant!
Apparently an essay by Francis Bacon, "Of Simulation and Dissimulation", appears to be the most "popular" piece of literature that calls attention to the words and their meaning. The most common reference I could find after that was a reference to ancient Rome! ...and actually, upon further inspection, it turns out that it was indeed the essay of Bacon himself that contained the reference.
So indeed, its connotation will mostly be related to the connotation of its more popular related words and synonyms, which appear to be "half-truth", "hypocrisy", "dishonest", concealment, and so on. It will tend to be negative, though just as "subterfuge" can be meant as a positive thing, as in spycraft, your usage will more strongly influence its conveyed meaning far more than connotation of the word.
But to be fair, most people just will have no idea what you are saying unless its very clear from context, because its not at all a commonly known word.
Dissimulation is associated (in my mind anyway) with two verbs that have very similar definitions, according to Merriam-Webster's Eleventh Collegiate Dictionary (2003):
dissimulate vt (15c): to hide under a false appearance [smiled to dissimulate her urgency —Alice Glenday] ~ vi: DISSEMBLE
dissemble vt (15c) 1: to hide under a false appearance 2: to put on the appearance of: SIMULATE ~ vi: to put on a false appearance: conceal facts, intentions, or feelings under some pretense
I leave it to your judgment whether any of these meanings has a positive or neutral sense; there is certainly a strong element of negativity in most of them. Arguably, the "false appearance" in Alice Glenday's quotation, though in some sense dishonest, might qualify as a social grace.
Perhaps most confusingly, Merriam-Webster's definitions seem to suggest that dissemble can mean either dissimulate (per meaning vt 1) or simulate (per meaning vt 2)—so by extension, dissembling can mean either dissimulation or simulation.
The first OED definition is as follows:
'The action of dissimulating or dissembling; concealment of what really is, under a feigned semblance of something different; feigning, hypocrisy.'
The inclusion of 'hypocrisy' makes it difficult to put a positive spin on the word.
I don't altogether agree that it is a word that has fallen out of use. As far as I know it is not one of the words that has been held to be unparliamentary, which one Member cannot use of another in the House of Commons without censure. In other words it can be a less impolite, and hence more gentlemanly way of calling someone a hypocrite.