I believe that PetitModem and other such programs are your friends.
Writing programs on a pc and uploading them to the petit computer 3
Root / Talk About Programs / [.]
ShiyasonCreated:
Yep. P/SEND is my personal favorite, although you need the proper cables to be able to use it. http://myu314.org/studyroom/product/pisend2/
http://www2.u-netsurf.ne.jp/~ozawa/petitcom/petiteditor.html
Text editor direct to wave transfer, no download.