I would just create a new Asset account and periodically update it to reflect the predicted lump sum's value. You would start the account with an opening balance transaction. Then you can either just modify this transaction (the account would always contain a single transaction), or periodically add "adjustment" transactions to reflect changes in the value, plus or minus. Depends on how much detail about changes you want to retain.
Markus.....what do I do now with this big pension asset account I have? I put my husband's paycheck pension deductions into it and now that he is retired and getting monthly pension checks I have a big asset account that isn't getting reduced. I finally realized that account is inflating my Net Worth and I need to get rid of it. Should I just make an adjusting entry back into the same account so it disappears into cyberspace? I guess I should have been expensing the pension deductions all the time instead of building them up in an asset account?
The OP seemed to understand that he had an asset and I didn't see any overt reference to a "lump sum" option.
From the OP-- presuming I'd add the updates manually and that I would take a lump sum.