We want to be able to setup default dimensions for GST transactions separate to the transaction line they are calculated from.

Users who want to set the default dimension value to be a specific code for GST accounts currently cannot do this, as the dimension value gets written to the GST line of the journal based on what is selected for the revenue/expense line. If you set the G/L account for GST to have a default dimension value and set it to "same code" to enforce it, it simply results in an error trying to post a transaction/invoice where GST is involved.

You should be able to have a different Dimension Code/Value on the GST G/L Account to the Revenue/Expense Account used in the same transaction.