> This is a good idea but I have to think how to implement it.

Hmmm, grub has no piping and no while read... has it? :)
What about serial --id <id> --eport and only export the config of one of the lines at a time to a predefined set of variables?

> The problem is that several simultaneous terminals may be active and in "usb" branch even multiple serial ones

Ha! Multiple simultaneous serial terminals - great! I have a terminal-server for remote-management and a local VT320 terminal here, hurry up ;)

It would be great for grub.cfg scripting if there were a way to read-back once configured parameters from the serial command.

For example:
If there was a way to read out configured serial parameters, it would be possible to generatate console= configuration items for linux and/or memtest86+ based on grubs serial configuration.

I'm sure there are other examples for other commands where a similar functionality would also be great (like terminals configuration, like RAID configuraion, ...).

Of course, in this special case it would be possible to store the parameters for the serial command in separate variables, configure serial with them, and re-use the variables later for other configuration items. I'm not sure whether or not this is the "grub way to do it". It would just be easier to be able to read the configuration back from the serial command than trying to keep variables and the real configuration in sync.