I used version 2.6.2 of darktable in this example.
You will need to create your signature as a SVG format file and put your signature file into the correct darktable folder ($HOME/.config/darktable/watermarks). Then use darktable's Watermark tool to add your signature to ONE of your images. I don't have any experience with creating watermarks, so my sample image below just uses one of the example watermarks darktable comes with. I positioned the watermark to the bottom right of my image, and chose to scale it for a smaller border.
Then use darktable's Framing tool to put a frame around your image. My first example image is a full frame image from my K-1. The frame I created for this image has an 11x14 aspect ratio instead of the 3:2 ratio of the image that came from my camera. To do this is a bit tricky, so here's the Framing settings I changed:
- Border size (I used to get a tiny side border for image cropping by printer): .8%
- Aspect: <click dropdown for list of aspect ratios - you won't see 11:14 in the list, you need to key it in> 11:14 <press Enter> (or whatever aspect ratio you want)
I also made the frame an ugly gray so the frame shows up on the web page. There are other settings that allow you to fiddle with the positioning of the image within the framing, but I have not played with them much.
So... at this point, all you've done is added your signature to one image. So...
Go back to lighttable view, make sure the image you just changed is selected. On the right side of the screen in the History Stack section, click on the Copy button. You will be asked what items of your history stack you want to copy. For this example, I deselected everything but Watermark and Framing.
Select your target images and again in the History Stack section, click on the Paste button. Your Watermark and Framing changes are then applied to all selected images. For the record, in my testing I made all my original Watermark and Framing changes on an image in portrait format. I then pasted my changes to images that were in both portrait and landscape formats - and even one square image - and it worked* for all images.
...I think this is pretty close to what you are wanting to do...
Copying the same history stack as I used in the first example, I pasted it into a Q7 image, which is the second image below. This Q7 image is cropped (in both dimensions) into a 16:9 aspect ratio so this image has a different aspect ratio than the first image (a K-1 image with a 3:2 aspect ratio). While the JPEGs I am have linked to both have the same dimensions, the raw images they were created from were very different in size. Also, original changes I copied from were in a different darktable folder than the destination images, so it is possible to copy watermark and framing settings from one folder to another.
* "Worked" in this case means that I got the correct aspect ratio frames and a watermark in the bottom right. However a 3:2 image in portrait format in a 11x14 aspect ratio frame ends up with most of the white space of the frame on the sides of the image. To fit the signature at the bottom you will need more border there. Everyone will have a different idea as to how they want their framing and watermarks to appear, so there probably cannot be a one-size-fits-all frame that works for all portrait, landscape and square format images.
These images are some that I pasted my history stack changes to: