Actually it's looks quite easy, under the Penalty Fares Rules:
We now know that, unlike Permits to Travel, this Promise to Pay only acts as "other authority" for cash payments, so in other cases people should presumably just rely upon rule 7.3 as the appropriate ticket was not available.
7.3 An authorised collector must not charge a penalty fare under rule 6.2 if any of the following circumstances applied at the station where the person joined the relevant train.
a There were no facilities available to issue the appropriate ticket or other authority for the journey which that person wanted to make.
We now know that, unlike Permits to Travel, this Promise to Pay only acts as "other authority" for cash payments, so in other cases people should presumably just rely upon rule 7.3 as the appropriate ticket was not available.