it would be useful because we have clients that if the sum of the work order is over a certain amount, we have to then produce an estimate/ quote for an escalation code before we can invoice.
Although it's unlikely we would add functionality to cover this use case, there is an alternative already available. We would recommend creating a non-posting invoice for this scenario. The basic steps would be:
Create a new invoice template and rename it to show "Estimate" or "Quote" (or whatever name is suitable for your customers).
When created, right-click the invoice and Mark Tax Invoice as Pending. This will create a non-posting invoice that doesn't affect your financials.
nb. For this template, you should remove the Status Stamp from printing.
** This also allows you to use the SO to "pseudo estimate" workflow.