DP has this feature and it is useful.

If this FR is not implemented I would suggest that REAPER's defaults here could be improved in any case.

6/8 should click on the 1st and the 4th eighth, likewise 9/8 and 12/8 should click once every 3 eighths. - this is standard musical behavior.

ummm... that behavior is standard for classical western music. Actually if we can choose clicks which beats of measure it clicking would be wonderfull.
E.g. Fine Metronome metering options. http://www.finemetronome.com/

sorry for bad English.

yes, my point is that as this is standard, that beats every third eighth should be the defaults for 6/8 9/8 12/8.

just to make it clear to the devs:

6/8 means two groups of three eighths each. 3/4 means 3 groups of two eighths each. etc.

so the flexibility for most usual click patterns can be achieved just by setting the defaults correctly.

for odd meters, more is needed though. a 7/8 bar might be counted 3+2+2 or 2+3+2 or 2+2+3 ...