Erel beat me to the punch, but I was 75% done and perhaps there's something useful here anyway, so:
Encode64 needs 64 distinct characters to encode 6 bits. 26 uppercase letters and 26 lower case letters and 10 digits = 62 characters, so two extra characters are needed. The B4X EncodeBase64 uses the MIME choice of "+" and "/", and "=" for padding at the end if there is not a whole number of 3 byte groups.
All three of those characters should be safe for use at the end of a URL, after the "?"
here-come-the-parameters marker.
If you wanted to be sure, you could replace the characters with something else after encoding, and ditto in reverse before decoding.
Or there is this in StringUtils, maybe that's something to look at:
Or you could write your own EncodeBase52 that just uses letters, or EncodeBase32, or EncodeBase16... no, wait, that last one's already done: