Is a skylight an option? You could put it over the aisle between the toilet room and the shower. They're not cheap but neither is glass, really. Otherwise, I think the pony wall will work. Have you thought about full bypass doors for the shower? There are some very nice ones out there.

Ooops. Sorry. I missed that you plan to have a bench. I guess that wouldn't work with a bypass door. You could forego the built in bench and use a free standing teak bench. That would work with a bypass door.

Glass will be a couple of K, but then so would a skylight. Not really anything in the "inexpensive" category.

I guess I should revise that from "inexpensive" to "least expensive." Skylight is not an option anymore, just thinking about something that can be done with the interior walls. Thank you for the suggestions!

In our MB we have a pony wall topped with glass between our shower and tub. It definitly lets light into the shower but the glass was a bit pricy. In our previous house and in my daughter's house we had a solar tube installed in a windowless bathroom and it lets in an incredible amount of light. It was much less expensive than a skylight and/or shower glass. We had them installed professionally and it was very quick and easy to do. My DD has had hers for 3 years now and there are no issues with condensation or leaking. I believe hers is a 10 inch tube. The picture below was taken in the daytime with no other lights on. It almost looks like there is curved light fixture mounted there but it is just an optical illusion because of the sun light coming through. In reality it is just flat like a recessed can light. Her sola tube was approx. $500 installed.

If you do a google search with ther term gardenweb and solar tube you should be able to find lots of information on them. I noticed on the Solatube website that they have now have some that are combined with an exhaust fan for use in bathrooms. They also have options with LED lighting in them.

Here is a link that might be useful: Solatube