Subtotal line item subtotal custom currency field

In some cases we are using multiple "subtotal" line item in our quotes, sales orders, etc. This works for the native out of the box rate and amount fields however we have some other custom column fields on these transactions but the subtotal line item does not subtotal these custom fields.

Also we can have multiple subtotal line items on the same quote/sales order because we like to break the line items up by distinctive characteristics such as class, vendor, etc.

My questions is how do we get the custom column fields to subtotal when we use the "subtotal" type line items?