UKAHFT can only really reply as if you were going to shoot the UKAHFT national shoots as affiliated & non affiliated clubs don't have to follow any rules or guidelines we set.
So lets assume you were attending a UKAHFT national shoot...... the first thing we'd have to do its discus with the hosting club to see whether or not the course is suitable for your safe access/egress as everyones safety is paramount & the nature of the sport means we are shooting over terrain & in positions that may not be suitable for a wheelchair
As to you modifying your chair we wouldn't have a problem if you fitted a post sticking up to act as your peg..... Ideally we'd need to meet you so that we could decide how you would be allowed to take your positional shots & also if the height of the pole needed to be limited but we would consider the following:
Prone - holding the pole using any body position in the chair
Unsupported standing - arms unsupported on the chair
Supported standing - touching the pole & any other part of the chair
Unsupported kneeling - only the leading arm resting on the chair but not the pole
Supported kneeling - only the leading arm supported on the chair & pole.
Hope that helps