If I digitally signed a file, I would use GPG. GPG can sign any kind of file, not just a DWG. You don't need the application that you open the file with to verify its signature. I'd be leery of the signature made by AutoCAD, because experts at designing CAD programs may not be experts at cryptography, and bugs that seriously compromise cryptography (such as using the wrong kind of RNG) may have no effect on CAD at all.
Regarding the OP. I would put the Architect and New owner on notice that I am aware that they have the file and if they use it they are to pay me for the rights. Ha-ha-ha. That has only happened once or twice; payment that is. I have on occasion when the reviewing jurisdiction is one that I frequent, I've made them aware of the possibility that parcel X may be going to be subject to a permit and that they may want to check the survey data source. That has resulted in more than a few Architects/designers snorting up their soup. Nothing like having to have a survey after you designed the project and the clock is ticking faster than mine.
The way I understand it, and I may be wrong, is that Washington has three ways to transmit survey maps/data. Hard copy with a stamp and signature. Electronic image (pdf, bmp, etc.) of the hard copy with stamp and signature. And a Digital copy which a CAD file would fall under. The Digital copy must bear a digital signature that is administrated by one of two firms licensed by the BOR and also an image of your stamp and signature. Problem is that currently, both firms have ceased doing business in the state. The BOR is exploring the idea of being the facilitator of the electronic signature program themselves.
My clients get two hard copies and an electronic image of the hard copy. I don't provide CAD files to my client unless they are another professional. I will provide them to my clients consultants only after I speak with them and am convinced that they understand my risk. They get hard copy and electronic copies along with a "courtesy" CAD file. I don't ever include my stamp or signature or data points in the CAD file and request that they advise me first before they (the consultant) share the file with anyone. I also provide a respectable disclaimer embedded on the drawing that they have to turn off in order to see/use the lines below it. I know the risks and have tried my best to mitigate them. I am also seasoned enough to remember (pre CAD) a certain architect and land surveyor who cut my stamp and signature out of a blue lined print and pasted it onto another modified one and got a permit.
I understand that I may be in violation of the rules of my BOR with regard to transmitting CAD files. They also understand that they cannot enforce something that they cannot do themselves.
Finally, one of the beauties of the recording system (for boundary surveys) is that when a client, their realtor or consultant calls me months or years afterward asking for a copy of the boundary survey I can point them to the record on file. That way I am not disseminating dated information.