Personally, I would prefer to specify the commands on a more "immediate need basis". Marking it out at beginning of tiddler means you either must know it when starting or you must jump up there again, put the mark and go down again.
...and if the parser can recognize this initial dot and the following dispersed markup commands, why can it not simply skip the initial dot step?
I (VERY much) like the idea to mark out splits for transclusion, but the user should not be forced to invent titles. I propose that if no title is inserted, then the current title is reused but with a number after (just like New Tiddler 1, New Tiddler 2 etc).
Is there a standard notation symbol for "split" or alike, that can be reused here? What do pro editors use? "->" is OK I guess
Or how about taking advantage of the standard heading markup; "!", "!!" and "!!!". These would probably often pretty natural places to split. For this case, an initial indicating command (e.g the dot) would make sense) OR there is a notation that takes advantage of the established "!", such as perhaps "!>" "!!>" etc. This would also make it easy to add splitout to an already existing document that uses regular headers.
Further, IMO there should be an (optional) "end splitout here" mark. If there is no such, then it continues until either the next (begin) splitout mark or to the end of the tiddler.
"End splitout" could perhaps be ">!" or "<!".(...which, btw, would open up for "nested super splitting" by adding the ">" to the different header levels in a document. One long document that has headings that include subheadings woudl split out first the top level and then traverse down in each new tiddler to split out sublevels. All in the name of tiddler philosophy! ;-) Seriously, don't let this last bit discourage anyone. It is something that could be implemented later.
It is also worth considering if, on split, it should be possible to add metadata?
<:-)