What is SDF text format?  Judging from the description, it is a
fixed-width field layout - with or without delimiters on fields?  What
about record delimiters? Newline?  What about escape conventions?
What about variable length types (VARCHAR - and BYTE and TEXT and so on).

No.  For any of a variety of reasons - depending on your level of
sophistication with DB-Access or ESQL/C programming.

Consider using ACE from Informix-SQL.

You might be able to batter SQLCMD into doing what you need (-F
fixed).  Not sure about the leading zeroes in SQLCMD - you could hack
it easily enough (output.c, array fmt_f); you'd have to decide on how
to integrate it smoothly into the program.

No.  You should probably just unload the tables from dbaccess and feed
them to Perl scripts to do all that reformatting.  If you don't know
Perl, then this is an ideal project to start learning it with.  It
will pay off in spades for you if you have to do a lot of data
reformatting like this.

Another alternative might be to dump the tables to XML and use an XSLT
processor to reformat the data... you, of course, will have to figure
out how to dump the tables to XML (I don't even know if/how Informix
J/Foundation can do that) and code the XSL script (which isn't nearly
as pretty as a Perl script) and feed it all to Xalan.