Assuming Panasonic hasn't done anything proprietary with the thread pitch on the filters, you should be able to use holders like the Cokin system using a 52mm holder ring. Usually, most of the issues with filter holders and such vignetting is with wide angle shots, but I suspect that the adapter tube is going to be more of an impact than the filter holder. The rings for the Cokin system are about 1mm thick, so not very thick. You probably want to get the P series if you do, but I'd advise trying the ring out in the store before buying, just to be sure that you're okay on threads (I only say this because they list filters as accessories designed to use the adapter tube).
As an additional note to the Cokin system and ND filters... Stacked ND filters tend to result in a strong magenta color cast. Just be aware of that if you do try them out and shoot RAW for better correction. This is an issue only for Cokin as far as I know.