Are there any effective estimates for up-rating resistors power capability by adding airflow?
What Ignacio said :-).
There is a large amount of material available on forced air cooling of heatsinks / plates/ surfaces and some of it is generally applicable. 
But the 'easy' way to get a feel for this would be to run one at near rating in still air and measure temperatures with an IR thermometer and ideally also contact thermometry (unless you are really confident in your IR-man-ship) and then fan cool and see how the temperature changes. 


Their chart on page 3 of the data sheet shows you that you WANT to run then at 25C ambient and that ANYTHING above that requires derating ! I infer from that that they are targeting Tresistor &lt;= 350 C and that thermal resistance is 3.25 degrees C per % of rating. 

I'd expect that if you shrouded them and blew air in 'goodly quantities' over them that you'd be able to achieve 'several times' the ratings. Nice and precise terminology :-).   

BUT what are  the resistors for?
There may be cheaper / better / easier ways - choose any 3.
Suitably sized air coiled Nichrome wire is wondrous stuff.
You can run it at a dull glow if you must.
(I have a friend who reports accidentally running 30 kW of it at a bright glow once :-).)