Just for full disclosure, this article is also relevant, in which shock-tube studies in the 50s and 60s were alleged to be contaminated by H atom impurities, which catalyze the exchange reaction in the gas phase. http://scitation.aip.org/content/aip/journal/jcp/79/6/10.1063/1.446178
These authors conclude that when measures are taken to eliminate such free radical generation, a direct H2 + D2 to 2HD exchange convesion is "highly doubtful", even at relatively high temperatures.
So, I think we can agree that while the (concerted) reaction might be thermodynamically favorable, it is likely pretty slow... and the rate also difficult to measure. I put forward that this may be the case because any transition state in a concerted reaction would have to be a very highly unfavorable 4-center structure, so the probability of a successful reaction is quite low. Of course, any mechanism which can give rise to cleavage or otherwise generate free hydrogen/deuterium radicals - such as photolytic cleavage, as you suggest - would provide a more convenient route to completion of the reaction. So, I would agree with you that, based on this cursory examination of the literature, the rate of a concerted reaction at room temperature is practically zero, but that other, possibly non-thermal mechanisms will still give rise to isotope exchange.
To the thread's main question, I think putting an actual number on a rate constant will be rather difficult. It'll depend on factors other than just the temperature and pressure of the gasses. This is not atypical of gas-phase reactions, which can often end up being far more complicated than they would appear based on a simple balanced equation.