I'll second the trick with the needle on the base side of the film. The way I've learned to do it is with an abrading tool designed for the purpose held perpendicular to the film, and just prick the base side of the film in a random way over the pinhole.

Negative retouching brushes--my new favorite retouching brush is the Richeson 18/0 Minature Series 9131 made in the UK. It's tiny, stiff, and keeps a nice point. It's definitely worth taking some time at an art store and checking out watercolor brushes for retouching. You're likely to find something better than the brushes sold in photo stores.

The Adams Retouching Machine illuminates the neg or transparency from behind, holds it in a rotating cradle, has a place to rest your wrist, can be used with two types of magnifiers, and will vibrate the neg at different speeds for pencil retouching and abrasion. I've seen them will long roll holders for high-volume retouching of school portraits and such. You can find a picture of one at www.veronicacass.com. Used ones can be found for around $50 often these days. A new one is around $600.