I really dig the hobo signs, they were fascinating to me when I learned about them when I was a kid, and still are.
There is a bit of a modern movement of new symbols
http://www.cockeyed.com/archive/hobo/modern_hobo.html with symbols for things like ticket scalpers, good dumpster diving, well stocked bathrooms, open wifi network, used frier oil, gps misdirection, etc.. some of this modern code is fairly comedic rather than really serious. But then the old hobo code had it's moments if comedy too. Old time or modern the hobos and tramps seem to have a bit of humor in them.
On the topic of the OP's modified code. The big thing here is having a group all on the same page. For the most part doesn't matter what symbols one uses as long as everyone who needs to can understand them. Though the one problem with using anything close to what FEMA or other agencies use, is the possibility of confusing what they might do with your own group's symbols. So you would want it different enough from their symbols so there is no confusion. You might expect some FEMA symbols in a SHTF situation thsat has turned into PAW. For a time FEMA or other agencies might have been trying to do their jobs, and find survivors and assist in a clean up. So their symbols might be out there. As well as the FEMA symbols are fairly known these days, and others might use them as they work through an area.
But I do think the OP's idea of modified symbols for his group is a great plan. And something I would recommend any group of networked preppers either adopt or create their own versions of.