Using senkats diodes is there anyway i could use a rechargeable cr123 in a dorcy junior host?
they are 3.6-4.2v when fully charged, is there any resistor or anything i could use to bring the voltage down to a safe level for the diode after it is boosted by the dorcy's driver?
No problem, at least in my builds.
I have built (4) Dorcy Jrs with 16x Senkat diodes, (the "weaker ones" from the beginnning on the year),
and (3) with PS3 diodes (BR) and I am using RCR123's in all (7) of them. You do have to set the current with a fully charged battery or risk overcurrent when you do replace with a fully-charged battery. I have heard others warn not to use Rechargables with this host, but I have not had any problems so far.
I adjust the output by placing a resistor or pot in series with the positive side of the LD. Others have also stated that these hosts change thier driver-circuit designs on a regular basis, I have not seen this yet, all of mine have been the same and match the pictures in Kenom's thread, so you will have to check this out for yourself.
You can read my "output reading versus resistor setting" with a PS3 diode in the following thread, in the first post. ;D The output did not change a lot with different batteries (2.8V-4.1V), but enough to take into account.
For a 16x red, as I remember I usually placed a 5-ohm resistor in the Dorcy, which delivered about 230mA with a 120-130mW output. Individual results will vary

.
Additional note, the higher voltage batteries do not seam to up the "max-boosted voltage", seams to still be the same, about 5v with no load, your actual voltage will match your LD, with either battery.
Dark Horse