Just to close this thread off, in case someone else has a similar question...
I went with Print (Print to all printers) by EuroSmartz, as the less expensive alternative. It's server, "MyPrint", installed on my iMac to which my printer is connected via USB, seems innocuous enough. It works as desrcribed, however...
I really didn't know anything about these apps (which is why I asked this question) and I was under the impression that these apps worked by adding something to the "Open In..." menu, or somehow hooking into AirPrint, but unfortunately, since I didn't know what I didn't know, I didn't ask the question clearly enough to allow anyone to disabuse me of that notion. Apparently AirPrint is fairly locked down.
What all these apps do (including HP's ePrint) is substitute that app for whatever iOS app you might otherwise use, in varying configurations depending on product, whether it's the lower-capability version like Print, or more comprehensive — and more expensive — variations like EuroSmartz' Print n Share, or Printer Pro, etc. So you open (say) Print, and then enter a URL, or select a contact, or whatever other type of data that the app supports — which, like I said, varies, and has to be specifically supported by the app. There are shortcuts, e.g., adding a "z" to a Safari URL to open the page in Print, but it generally feels like a clumsy process. The "Print by G3" feature, which I haven't yet tried, looked attractive.
But it does seem to work well, within its limitations, and so would seem appropriate for many situations with older printers. To get full, intergrated printing, however, it seems you do need an officially-AirPrint-capable printer.
