The last I heard the IHR lump sum 34 weeks pay plus either 9 or 12 weeks PILON(taxable), depending on service.
You don't say which pension scheme you're in but I'm assuming you're in either section A, B or C of the RMPP and currently in the DBCBS?
The plan guide states:
What if I am absent from work and not being paid?
If you have a period of unpaid absence you may, on your return to work, pay all contributions (both your own and the Company’s) that were due during your absence, based on the pay you would have received had you not been absent. If you choose to do this, your Cash Balance benefit will be treated as having built up over the period you were absent in the same way as it would have done if you had not been absent. Reservists have three options as set out on the ‘Life Events, Taking a break from work’ page of the website.
In theory you can pay some of your lump sum into a Flexiplan AVC, but not directly into your actual pension. But as you're only allowed to contribute into AVC's via your pay, you'll have to set it up so it goes in the week you receive your lump sum.
I would advise getting in touch with the pension service centre in Sheffield for clarification on whether you can actually do that or not.
Their contact detail are on the RMPP website: https://www.royalmailpensionplan.co.uk/
Another option might be to set up a personal pension and pay some of you lump sum into that. It will be totally separate from RM, but you will benefit from tax relief being added.