Nothing to it (for somebody with the chutzpah to call himself "Programmer Dan").
Well, maybe a little. Your have to know the encoding for the values 0 to 63.
In general, encoding to Base64 is done by taking three 8-bit characters of plain text at a time, breaking those bits into four sets of 6-bit numbers, and creating four characters of encoded text by converting the numbers (0 to 63) to arbitrary characters. Actually, the encoded characters aren't completely arbitrary, as they must be acceptable to pretty much ANY method of transmission, since that's the original reason for using Base64 encoding. The system I usually work with uses {A..Z,a..z,0..9,+,/} in that order.
If one wanted to be nasty (which one might expect in the case you're dealing with), one might change the order, or even the characters, during the process. Of course, if you have examples of the encoded Base64, you can see what the character set is (unless the encoding uses more than 64 characters). But you still have the possibility of things like changing the order as you encode or decode (simple rotation, for example). But, I digress. The question is about searching for encoded text, not deciphering deliberate obfuscation. I could tell you a lot about that, too.
Simple methodology:
Encode the plain text you're looking for. If the encoding results in one or two equal signs (padding) at the end, eliminate them and the last encoded character that precedes them. Search for the result.
Same as (1) except stick a blank on the front of your plain text. Eliminate the first two encoded characters. Search for the result.
Same as (2) except with two blanks on the front. Again, eliminate the first two encoded characters. Search for the result.
These three searches will find all files containing the encoding of the plain text you're looking for.
This is all “air code”, meaning off the top of my head, at best. Some might suggest I pulled it out of somewhere else. I can think of three possible problems with this algorithm, excluding any issues of efficiency. But, that’s what you get at this price.
Let me know if you want the working version. Or send me yours. Good luck.
Cplusman