Unfortunately, in this case, I haven't seen a trace of him since. So if you live in Austin, particularly in Delwood/Duplex Nation/Cherrywood/French Place, please keep an eye out for this animal! Or if you know anyone who does, please send them a link to this post or this craigslist ad!
In case any of you are in a similar position, here's what I'm doing (and also let me know if you have any other ideas):
--The Austin Animal Center is designated as the primary receiving site for lost cats and dogs. They have an online database, which they update several times a day, of the animals they've received--strays or owner surrenders--so you can search for your pet there, or you can stop by during their open hours to look yourself. You can also file your pet's microchip number and photos with them, so they can track you back if your pet comes in later. (They also have a lot of adorable cats and dogs available for adoption, if you're looking for a new mascot.)
--Neighborhood listservs and newsletters (in my case, there are several; the email listservs are fairly well-subscribed).
--Posters and flyers--from what I understand, these are crucial because a lot of times a lost cat wandering around doesn't look lost, so people tend to tune them out as visual background. In my case, I'm putting up posters in the neighborhood spots, but also putting flyers in every mailbox on my street and the neighboring street, which is where he's most likely to be.
--Craigslist lost and found.
--Facebook and Twitter: another reason to make a Craigslist ad is that you can pass the link around via social media. I've been touched by how many people, even strangers, are happy to help spread the word about a lost pet.
--Making sure your pet has a microchip. From what the vet explained, most microchips can be identified by a universal scanner, and (assuming you've been taking your pet to the vet) the vets will have the number on file if you can't locate it. I think some companies register the chips back to themselves, so if you aren't the original owner of the pet, as in this case, you might not know who would get the call. But it's reassuring to know the number is on file, especially if your pet doesn't have any unusual physical quirks. (There was a chihuahua at the Austin Animal Center that was listed as "Markings: Red Sweater", which is kind of adorable.)
--Making sure you have recent photos of your pet, ideally outdoor photos or photos that show scale.
--Searching the physical premises and keeping an eye on your own yard (if you don't see your pet, you'll at least have a better idea of what other animals in the area are doing, and if any of them are new/aggressive).
Thanks in advance for passing this link along. I'll give any updates when they have them. Fingers crossed!