One of my buds back in school was hard of hearing.  He had both his bike and his car set up with small 12v solenods.  When the flasher was sending current anywhere, the solenoid would actuate, moving the plunger far enough so it would hit something with enough force so you could actually feel the little bit of impact in your hands.  

In the car, he had it mounted under the dash in a position to thump the steering column. You could feel it quite easily in the steering wheel.  For those of us with normal hearing, you could also hear it.  It was pretty loud inside the car, but that didn't help him any,... he needed to feel it.

On his bike (a Bonneville) he had it set up to thump against a fork or triple tree or handlebar or something,... I forget.  It was really easy to feel that "click" though, even on a vibrator like a Triumph.

The solenoids were pretty small,.. maybe 3/4" square.  I don't remember what they came off of.  I'll bet there are a thousand 12v low amperage small solenoids to choose from.

I don't think I'd like a beeper.  I'd feel weird making noises like that in traffic.

Routy...
I forgot to credit you for the piezoelectric
buzzer idea.
Thank you.
Took me a bit to figure out why this simple
solution actually works, but the electronics
principles are solid and it won't hurt your bike.
That Radio Shack buzzer I used may not be loud
enough if you have a loud exhaust or wear ear plugs.
But for $3.59 and a few minutes time it may be
worth a try for some. It's definitely not loud
enough to mount in the headlight (which is a great
idea) so I just taped mine to the flasher relay
under the tank out of the weather.
I have a few more installation pics if anyone's
interested.

hotprops:

Thanks for the heads-up on the buzzers still being available from Radio
Shack.
Unfortunately, so many of the stores are closed now, most of us will have
to mail-order them like you did.
The good news is that we can avoid the shipping charge by ordering
at least 60 buzzers at a time  

Just in case others are interested in the details of how I installed mine,
here are a few more pictures:

Strip some insulation from the buzzer wires:



Unplug the flasher relay  from the connector, push the buzzer wires into the
back side of the connector, then trim the ends off to about 1/8" or so.



Plug the flasher back into the connector, tape the buzzer to the flasher,
cut a hole in the tape to uncover the buzzer speaker opening, then push
the flasher/buzzer assembly back into the rubber holder thingy under
the right side of fuel tank.