Find text within an email in GMail or Inbox by GMail Mailing list digests are generally very long.  They contain multiple emails concatenated together with an index and my traditional approach to reading a single email on the desktop is to use Ctrl+F (find text) to navigate/locate content within the email.

Android browsers typically have a 'Find on Page' function, but an equivalent doesn't appear to exist with Gmail or Inbox by GMail.  Have I overlooked something?

Alternatively- are there any apps that offer a generalized search mechanism for selectable text regions in Android or extend the functionality available from a long-press on selected text?
You can search for emails containing a certain bit of text as stated here but that doesn't narrow things down to the specific portion of the page. However, the search functionality in Inbox will highlight the section of text which is a step up. Depending on your device, I recall having this functionality on my stock S4 ROM, so if your phone has another email client try that out. 

I haven't seen this functionality or anything similar in Outlook or the Yahoo client. 

So I'd say your best bet to meet your Ctrl+F type searching is through the browser, then inbox isn't too bad as it highlights occurrences but doesn't jump to them then the rest do the same thing of searching for text in an email but not highlighting it or do anything like Inbox or a browser can do. That being said, I haven't tried many other email clients on android but that's what I got from the ones I tested.