I’ve written up what I did to set up my machine to write Swahili in Arabic script, and the result is contained in this howto document. The keyboard layout file listed in Annex 1 can be downloaded here. Any corrections or additions are welcome.
What has actually surprised me is that it is almost as fast to type Swahili in Arabic script as it is in Roman, and it takes much less time than it would to add all the diacritics to make a proper transliteration of the manuscript.
I’m not sure whether this has already been done for other African languages that have used Arabic script in the past (eg Hausa), but it seems a useful way of using modern technology to help safeguard an important area of cultural heritage.