You can already do this using #4 on the numpad, making unlimited of the selected item. Just drag and drop gold piles and keep making more.. It's a little more tedious than just hitting a button to have a ton of gold, but it works! Then just turn it off and stack them all up into one pile.
Why would you ever go through all that when you can just use the edit item option to set the gold amount to whatever you want? Just as with any item in inventory item (gold is an item in your inventory, it's under miscellaneous), select your pile of gold, move it to a new spot, alt-tab to the trainer, select the edit item tab, set the number you want and hit set. Now you've got tons of gold. Just use the character you selected to do your bartering and buying (or split the gold up between characters if you so choose).