If you know (or can reasonably guess) the limits of your retaining wall, you can add a key station to your corridor at each end of your retaining wall (and anywhere else) and I think that will serve your purpose. This option doesn't necessarily allow you to override anything; it just ensures that the template is applied at that specific location no matter what your template drop frequency is.
I believe that if you set the key station wrong, you should be able to adjust it in the same way that you can adjust the ranges of your template drops and all that.
Edit: I just realized that the base interval is established in your template drop element (not the corridor element). You could terminate your low-frequency (high interval spacing) template drop before this area and add a new, higher-frequency (low interval spacing) template drop over the range of the retaining wall (even if it's using the exact same template) and set the interval to something smaller, in case that's one of your goals.