I know that I can create a custom arrowhead in one file and use it in that file but is there a way to create a custom arrowhead and have it display as a choice in the Qleader>Settings>Arrows pull down as well as the Properties>Lines & Arrows> Arrows pulldown? Where are the standard arrows stored?
You have to ensure the block in INSERTed in your file, then it appears: there is no method for you to alter the default core offerings.
Try adding this as a startup routine in your ACADDOC.LSP file, or a macro at the start of buttons you may push on screen if you don't type, and ensure this block is already in your template files.
Have the arrow head as its own drawing and place that drawing in a folder on the search path for AutoCAD.