A friend came over and, astonished, asked - "Can you run computers in a workshop?" Well, obviously, they did run in a workshop without dying, for a long time, so I said "yes", but the seed of doubt was planted.
So I decided to check, and the first picture below is the result (that's Lian-Li V-Cool PC-V1200B, having worked in the workshop for about two years). The second picture, for comparison, is somebody else's computer.
Well, about a year later I've decided to upgrade the memory in P-182, and used this opportunity to check the effect of dust again. Below are the pictures.
In other words, the condition of the case is more than satisfactory. The only contaminated surfaces are the fan blades, that's it.
Too bad I lost a chance to photograph the filters - dust has fallen off as I opened the cover. I am surprised with the efficiency of these washable filters - they gather, in fact, more dust than usual foam filters.
One more factor that contributes to low contamination is the airspeed - the Scythe SY1225SL12M fans used in combination with active speed control offered by Asus Striker II Formula motherboard provide very low airspeed while maintaining decent cooling, if needed. I'm sure that older case with higher speed fans would've been contaminated much more than this one.