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You could have a script on before load that adds a custom field to your transaction and pulls back the relevant value and populates it. This would only ever reflect the current value. Just be warned it would not save the value on the transaction if you wanted that to happen. You could extend it to copy the temp field value to a regular field on before submit if that's what you're trying to achieve. That would be a snapshot and only update if you navigate to the record. If you need to keep it up to date (i.e. the custom field on the custom record updates regularly) you'd be looking at a scheduled process to update your transaction record at which point you don't even need a temp field just directly populate whatever you want to have that value on the transaction.