API 2201 does not prohibit it as far as I can remember, but the only 45 degree hot taps I have heard of are on flare lines.
If it has to be done make sure you look at the design calcs for the connection/reinforcement & make sure you take into consideration the weights of the valve and tapping assembly.
Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:
What size hot tap are you looking to perform on the 6" Pipe? While angle taps are not routine, you can contact several experienced contractors for this sort of service.
Here is a press release from Furmanite concerning an angle hot tap. They have offices around world that perform this service.