The DataMatrix article on Wikipedia mentions that it supports only ASCII by default. It also mentions a special mode for Base256 encoding, which should be able to represent arbitrary byte values.
However all the barcode generator libraries that I tried so far support data to be entered as string and show errors for characters beyond ASCII (Onbarcode and Barcodelib). There is also no way how to enter byte[] which would be required for Base256 mode.
Is there a barcode generator library that supports Base256 mode? (preferably commercial library with support)
Converting the unicode string into Base64 and decoding from base64 after the data is scanned would be one approach, but is there anything else?
it is possible, although, it has some pitfalls: 1) it depends on which language you're writing your app (there are different bindings fo different DM-libraries across programming languages. For example, there is pretty common library in *nix-related environment (almost all barcode scanners/generators on Maemo/MeeGo/Tizen, some WinPhone apps, KDE thingies, and so on, using it) called [libdmtx][1]. As far, as I tested, encodes and decodes messages contatining unicode pretty fine, but it doesn't properly mark encoded message ("Hey, other readers, it is unicode here!"), so, other libraries, such as [ZXing][2], as many proprietary scanners, decodes that unicode messages as ASCII.
As far, as I dicussed with [ZXing][2] author, proper mark would probably be an ECI segment (0d241 byte as first codeword, followed by "0d26" byte (for UTF-8)). Although, that is theoretical solution, based on such one for QR-codes and not standardized in any way for DataMatrix (and neither [libdmtx][1] nor [ZXing][2], do not yet support encoding with such markings, althought, there is some steps in that way.
So, TL;DR: If you plan to use that generated codes (with unicode messages) only between apps, that you're writing — you can freely use [libdmtx][1] for both encoding and decoding on both sides and it will work fine :) If not — try to look for [zxing][2] ports on your language (and make sure that port supports encoding).
1: github.com/dmtx/libdmtx
2: github.com/zxing/zxing