Any tricks out there to conditionally lock specific fields (not entire records or layouts)?

I don't know about tricks but security allows conditional restriction on fields. If you can provide an example, we can provide the logic on how to handle it. :-)

For example, if field "Tracking_Number" is not empty, prohibit user from modifying fields "Weight", "Shipping Rate", and "Ship Date".

But they should still be allowed to edit/delete the tracking number, as well as other fields on the layout/in the record.

Security has the ability to use the calculation engine to control record and layout access, but when it comes to individual fields, it's just basically on or off as far as I can see...

I tried testing for "Get ( ActiveFieldName )" in the record edit rules, but it seems it only evaluates when you first enter the record (so you can just click in a permitted field first).

you could use 2 different privilege sets to control which has edit access and read only. Another option may be to put those fields into its own table.

Put the fields in a popup window controlled by a scripted Edit button. The button will only open the popup window if validation passes.

Wouldn't it be cool if you could use the conditional formatting feature to override the "Field Entry" setting? (feature request sent!)

You could turn the fields into buttons and use a script trigger to allow/disallow entry based on validation.