Very old thread, I know, but sometime after this post, this software appeared:http://joyfulcoder.com/memopad/http://atariage.com/forums/topic/245258 ... computers/
It's for Windows, but might run under WINE. I know of no other ATASCII related editing tools (though I have not looked very hard). I did see some thread recently about providing an "IDE" to support 8-bit development, but I have no idea if this included BASIC.
There is also a Perl-based "detokenizer". Again, it's a converter not an editor.
off-topic Atari BASIC rant:
I too am also interested somewhat in writing code for the 8-bits, although personally I would avoid BASIC...
Assembler or C (Lightspeed C for example)have real world benefits in learning (ie, jobs), plus you can apply C and assembler experience to other processors like the Atmel micro, used on Arduino. Interestingly, the Arduino provides interesting opportunity for development of new Atari devices (such as the SD-card disk emulator devices, for example Ardiono2SIO).
Atari BASIC is my first language, but it's arguably not a very good implementation BASIC: no JIT compilation like what Turbo Basic gave us later, plus no bitmasking, not much direct access to Atari features like P/M graphics, and an awful editor (you could easily create a line too long to edit, for example). When you get close to running out of RAM (or you want something to run on the 400) you have to conduct all sorts of hackery on the BASIC code (creative variable usage) in order to increase free RAM, at the cost of code maintainability.
Atari since 1983 (1200XL, later ST). Now a Linux freak who uses a Mac at work.