Here's my take.
This is not an additional aircraft rating, so it does not fall under 61.63 (which is typically used to drastically cut the hour requirements for adding a category or class).
So it falls under 61.65 ("Instrument Rating requirements"). To break it down, you have to:
- receive ground training
- receive flight training
- get an endorsement to take the practical test
- NOT have to take a knowledge test (a7)
- pass the checkride
It then goes on to say what type of topics must be discussed in ground and flight training.
But then the big one - "(d) aeronautical experience for the instrument-airplane rating"
- 50 hours XC PIC, 10 of those in an airplane (rest can be in helicopters or airships or whatever)
- 40 hours of instrument time (sim or actual), 15 of which have to be training received from an Airplane CFII, this includes:
- 3 hours from a CFII-A within the 2 months prior to the checkride
- "Long XC" in an airplane - 250 nm, three airports, three approaches
Note that you will likely already have 40 hours or more of instrument time from your IR-H training and subsequent flying. So, logically, as long as you meet the XC PIC requirement, the minimum training you have to receive is 15 hours and the long XC, both of which do have to be done in an airplane.