A package that traces base functions

[brodie gaslam]

Apologies, this is a cross post from 'R-devel', where I sent this message a few months ago when I did not realize there was an 'r-package-devel' list.  So here it is again in the correct list:

I'm working on a package that implements a REPL.  A typical interaction with the package might look like:
> launch_REPL()REPL> 1 + 1[1] 2REPL> QDo you wish to save results? [y/n]REPL> ygoodbye ...>
This is very similar to what happens when in `browser()`: the REPL evaluates arbitrary R user expressions and offers some additional commands.
In order to implement functionality required for the REPL I must trace some functions in the base package.  The trace is removed `on.exit()` from the REPL, so the functions are only modified while the `launch_REPL` function is evaluating.  Unfortunately this is against the letter of the law (as per CRAN policy):
> A package must not tamper with the code already loaded into R: anyattempt to change code in the standard and recommended packages whichship with R is prohibited.
Is there any chance that this very limited (only during my function evaluation) modification of base functions with `trace` could be considered to meet the spirit of the law, if not the letter?  Package users would be duly notified this is happening.
Alternatively, if this is not allowed, would it be acceptable to ship the functionality turned off by default with a user option to enable it (documented with warnings about how this is non-standard behavior, etc.)?

Regards,
Brodie Gaslam.
PS: More details for those who care: the REPL among other things implements an environment that has for parent `as.environment(2)` so that objects in the global environment are not visible while in the REPL, but otherwise the full search path is.  Anytime the search path changes I need to update the REPL environment to re-point to `as.environment(2)`, which means I need to know when the search path changes.  I do this by tracing `library`/`attach`/`detach` and triggering a side effect that updates the REPL environment parent any time those are called.  The search path itself is untouched.  I cannot just parse user expressions searching for those functions as the user can use any arbitrary expressions, including sourcing files that contain the `library`, etc. calls.  
	[[alternative HTML version deleted]]