thanks for your inspiration, I now have a couple of options and will figure out what works best.
Some pro's and con's, which may be useful for others:
Using SUBNM in Server Mode:
[-] I had to create an "only-alphanumeric-alias", which I missed initially and felt this option would not work for me.
[+] Now with such an alias created, this works just great and even better: out of the box with Canvas
[-] In Server mode, I cannot create a radio button list SUBNM, because in this format the SUBNM does not provide a search box, of course. This is exactly what I tried to achieve in my example above, see screenshots.
[-] I have to implement kind of a "element click" functionality, which the radio button SUBNM brings out of the box
[-] I cannot use the filter directive of angular, as my dimension is huge and I cannot retrieve all the Elements in advance and then leave the filtering up to the front-end. It needs to be applied already in the MDX.
[+] (no pro using the elment list for my requirement)
[-] Not out of the box, so no simple tm1-ui directive. However, I could wrap it in my own directive to make it re-usable.
[+] Flexibility in defining layout and functionality
[-] Pattern search via MDX against alias names not so easy. I still Need to figure out the hint of cw-ch using INSTR function. However, if it is case sensitive, this might turn out as a no-go as everything else in TM1 is not case sensitive.
I think to proceed, I will go on with the SUBN in Server Mode and work on a custom directive when I find the time. I prefer to offer the look&feel as shown in my screenshot: type a few characters, resulting in a radio-button SUBNM list of matching elements close to realtime. This way, the matching results stay visible and the user can easily switch without going back to the dropdown.