I think there is still scope for your original idea not covered by the split bill function. The split bill option requires everyone to be paying an equal share but whilst travelling myself and friends will regularly pick up a bill on one card where each of us will want to pay different amounts.
We can use customise amount but as you’ve mentioned you then have to work out the home currency equivalent that you owe. Currently I’m calculating total € per person and using the MasterCard exchange rate page to work out the £ each should send. If I could request the amount in € based on the payment receipt I would be able to skip out the exchange step.