any ideas for overripe avocados, besides guacamole?

Drop them off at the local fire station/shelter with 6 limes, a tomato, onion and a couple of bags of chips.

Avocado pie! Tastes and looks like Key Lime, but is super easy. I use this recipe but with lime juice instead of lemon. http://www.californiaavocado.com/recipe-details/view/31487/california-summer-avocado-pie

Another idea is to make avocado crema sauce (avocados, sour cream, lime zest & juice, cilantro, jalapeno peppers if you'd like, thin out with some water and blitz it all together in a blender/food processor.) You can use this sauce for anything really.

I've used frozen (and then defrosted) avocados to make muffins! And they were delicious. Also chocolate cupcakes, also delicious :-)

Smoothies
Sandwich filler (instead of butter/cheese)
Avocado quesidilla (avocado instead of cheese)
Chocolate pudding (I would guess you can freeze this!)

If you really mean "overripe" rather than "very ripe", the compost pile. Some years ago I stayed with friends in Brazil who had a HUGE avocado tree in the back yard; they used to serve a whipped avocado dessert- I don't think that it had much but avocado and honey in it, but all kinds of spices, citrus flavors etc. could be added. Avocado is very good as a bank canvas- look at the hundreds of guacamole recipes out there.

18 is a lot! Avocado toast plus a fried egg anyone?
I mash avocado with Moroccan cured lemons when I don't feel like mashing and mincing a lot of separate things like garlic, cilantro and onion for guacamole. Sometimes we just eat a half in the shell with a wedge of fresh lemon and some salt.
Sliced avocado goes well with mild smoked salmon in toasted whole-wheat bread sandwiches or in a salad with hard-cooked egg, roasted peppers, and tomato.
I would also try mashing and freezing some (without seasoning). Remove any blackened areas first, of course.
During a difficult time last year, when people were sending meals to us, a family member brought us quite a lot of individually-packaged commercial guacamole. I froze them and packed them for lunches for the family. If it works for commercially packaged avocado, it may work for home-made too.

I made this smoked trout-avocado salad last week and loved it. I did not put it on toast--just served it as a salad--and grated some fresh horseradish into the mixture. The recipe is a keeper.
http://food52.com/blog/16832-let-smoked-trout-some-toast-save-your-dinner

I have never tried this idea because I don't have an ice cream maker, but an avocado-rosewater ice cream intrigues me. One day I may buy an ice cream maker just to give the idea a try.

This creamy avocado dressing is really tasty: https://books.google.com/books?id=CIZKCgAAQBAJ&pg=PT178&lpg=PT178&dq=creamy+avocado+salad+dressing+recipe+cooks+illustrated&source=bl&ots=DUtigz5cFb&sig=JiWFPMtixIejkkngcx-89CnEBqE&hl=en&sa=X&ved=0ahUKEwieoubEr-_MAhUC42MKHWbGCgEQ6AEIUTAH#v=onepage&q=creamy%20avocado%20salad%20dressing%20recipe%20cooks%20illustrated&f=false

Yes, make it into a spread and use it on toast or a rice cake. Mash the avocado, add a few squeezes of fresh lemon or lime juice, salt, pepper and any other spice/herb that you enjoy. Top it with grated cheddar for a quick breakfast or midday snack. Possibilities are endless!

Maybe a smoothie?

I think much of this depends on whom you are feeding. Single? Childless couple? Well, a couple of avocados might go a long while...

Parent with three teenagers? Well, guac might last three hours, about as long as they're in the house for one spell.

Depending on whom you are trying to feed, freezing excess guac might be a strategic approach to your oversupply situation.

Good luck.

On tacos, in a salad. I've read about putting them in smoothies but haven't tried it yet. Make chocolate pudding with them (if I remember correctly, you mix avocados, tofu, agave and cocoa powder together).