You really want the aperture at the waist of the beam. I don't see how to define the speed (f/#) otherwise, and an aperture before the front element will simply vignette. If it's an inexpensive lens then I'd say go ahead and do the exploratory surgery and try to install a waterhouse stop. Waterhouse stops have the (tremendous) advantage of giving you a very smooth aperture even when you're not shooting wide open... so the bokeh may delight you. I mean, I ripped the aperture blades out of an anastigmat and I am glad I did.

How would you define lens speed if the aperture is in front of the lens? :?

P.S. I can imagine putting an aperture at the conjugate focus but, offhand I don't think I have seen this done except for microscopy.

P.P.S. Aperturing the front element might enhance contrast. But I don't see any other benefits.