Hi,
I'm not sure if this is still an active issue. I have been working on a similar issue were I needed to refresh the Incident detail form after an update from a custom activity.
I also thought that the KEEP.IsModified would solve it, and it might be that this has to be a part of the "custom activity form".
I did manage to refresh the Incident detail form by adding this to the custom activity form:
if ( typeof parent.opener == "object" && parent.opener != null &&
typeof parent.opener._dtl == "object" )
parent.opener._dtl.tabReloadOnSave = "";
I added this just before the line:
ahdframeset.top_splash.next_persid = "$args.persistent_id";
Seems that if you don't add this then the tabReloadOnSave in the parent-form will have the value alg if you used the normal way of opening the activity form (popupActivityWithURL)
Give it a try :-)