Can I place a hold on funds against future refunds on my P24 balance?

Yes, below are the details for transferring the balance to be blocked.

Recipient: PAYPRO S.A.
Account number: 67 1140 1010 0000 5402 7000 1006
Title: Balance lock, IDXXXXX
(where XXXXX is the account ID in the system)
Amount: X.XX PLN