If you inspect the transaction (add to the url &xml=T) look for a recmachcustrecord.
You should be able to identify your custom record sublist.
At which point you can acces the list such:
<#list record.recmachcustrecord_mysublistrecord as recmachcustrecord_mysublistrecord>
As you would with the item list. (obviously switch out the correct sublist name!)
There may be limitations to this depending how your sublist is added to the transaction I'm not 100% sure (it's 1:36am and I'm half asleep 😄 )