As an update to those interested: the noise was not from the chain or tensioner, but rather improperly adjusted valves. The dealer I took the bike to here in San Diego, Fun Bike Center, adjusted the valves to the specification of the WR250F, 2008 model year, for which the Yamaha manual cover shows "WR250FX." In other words, the valve clearances on the exhaust valves were far too tight. As a reference, the 2009 WR250F manual cover shows "WR250FY," indicating that these manuals definitely refer to the 5-valve enduro motor. In tearing everything apart to replace the tensioner as I had planned, I checked the clearances to find the intake side at 0.15mm a piece and the exhaust at 0.18mm a piece.
As a word of caution to others out there: make sure that if you have your valves adjusted by one of these mainstream dealers with an utter lack of attention to detail, they don't use a 2008 WR250F manual for your specifications. There is clearly some confusion in the cover of a manual reading as "WR250FX."
I promptly took the bike back to this dealer demanding proper valve adjustment. I'm hoping that my exhaust valves were not damaged or burned in any way during this 1500mi/2mo space of time I rode the bike like this.