A "tax table" field in the register would be helpful so we don't have to use vendors/bills etc or manually add tax splits.

I would love to be able to pick a tax table for each split for calculating Australian GST. This would also be useful for creating GST BAS reports. I need to report totals for GST inclusive/exclusive sales, GST inclusive/exclusive capital purchases/non-cap purchases etc. These can be worked out from my GST accounts on the balance sheet, but with a tax table field at the split level I could generate lists for each GST category using that field.

Can't you create the following 'vendors'?
Cash Purchases
Credit Card Purchases
Cheque Purchases
Or even
General Purchases
And then enter the item as a Cash Purchase?

I don't know about you, but in my neighborhood every city has a different sales tax rate schedule that applies differently to, say liquor, food, gasoline and labor. The tables change frequently as "ins" are voted out and replaced by new "ins"