Why would a malicious app bother opening other malicious apps if it can do whatever it wants
all ready? You're missing the point. The only purpose of the file scanning talked about here
is to detect malicious software when it's installed/downloaded/saved on disk. When you have
malicious software doing whatever it wants you've already lost. "Scan on open" isn't good
enough to prevent malicious apps from writing other malicious files anyway. For more details
read the lkml thread.
Shell scripts aren't statically linked apps at all, it's just the shell running, in general a
dynamically linked bash, so LD_PRELOAD will work for them fine.
We're talking about damn virus scanners here, not a security framework (The former is mostly
about detection, the latter mostly about damage mitigation). If you want your own brew of
security then write an LSM module, or SELinux ruleset, but if you want to do something as
simple as file scanning then just do it with a preloaded lib.