Ended up working this out a little different way... added the columns that are being filtered in the line FC to those of the point FC (was nice to have them there for other reasons too). When the app loads if fetches all the line layers where clauses and then when the legend changes it dynamically scans all the visible sub layers and ORs together all their definition expressions. Then it places the expression on the point service. Works pretty good.I placed a little sync icon next to the point layers that get dynamically filtered to let users know, and wish each ArcGISDynamicMapServiceLayer had a refresh complete event or something to make them spin while loading. I hooked the icon to the Map.Progress event which is close enough, but would be a nice enhancement for the service layers themselves (like FeatureLayer has).Still would be curious if anyone has any better ideas... thanks!