I would use backups (nightly incrementals) to cover op system, application
code (the installed oracle code, etc...)
and I would use "archives" to save all the oracle .dbf files...
This way (as you said) all the open/active oracle files are excluded from
nightly incremental processing.
OH, the TDP product is specifically written to do all this but if you don't
want to spend the money for that
you may either maintain a list yourself of all the .dbf files (and anything
else you might need)
or write some scripts that issue oracle queries to list all the files
associated with tablespaces and go from there...

you can't really yoyo the include/exclude list like that because as soon as
you exclude those files from backup and run one of your weekday backups,
any/all excluded files are expired if they exist !

hope this helps
DWight