....I figured a helicoil would go in, but was afraid the brass washer would not seal properly with a helicoil in place...
That is a valid concern. I believe that the metal washer is broad enough to cover the increased diameter of the drain hole cut by the helicoil tap. The challenge is to insure that the helicoil threads are cut perpendicular to the gasket surface. When doing this by hand, I use a steel rule to check the tap alignment each ½ turn of the tap. This application would be especially challenging because there isn’t a lot of surface area available to work with; additionally the plating process may have compromised the gasket surface of the fork slider.
The other thing to consider is that the drain hole is fairly shallow, so it is likely that the helicoil will need to be trimmed to length. Count the threads cut by the tap and trim the helicoil to one less in order to recess the insert below the gasket surface. Trim the insert from the top with diagonal cutters, one thread at a time until it is cut to the appropriate length.
In this application it is likely that there may be a slight alignment issue and the drain plug may not sit perfectly square on the metal washer. In this case the use of RTV sealant or substituting a fiber washer may help to insure a positive seal.
djkak