Adapative or otherwise variable lengths can be implemented within what otherwise is a series function ...via knowning what you're doing. This problem is not unique to TS. It's unique to series functions. If Multi-Charts includes series functions then MC suffers from the same limitations. Most programmers will tell u it's impossible but that's because they're stuck, unable to think for themselves, at least in this area. But the solution is simple.
Write a series function that stores the results of all the lengths u could ever possibly need, where every series function call is replaced with inline code. If currerntbar changes the length from the previous bar then the series function accesses the most recent stored value for the new length rather than the stored value for the old length. If u don't want to store all that data, a separate solution would be to loop thru the necessary history to prime the inlined version of code that replaced the series function calls...but only when the length changes. If the length remained the same on some bars then it wouldn't have to loop thru all that history (on those bars). IMO that's a lesser hack but ymmv.
I already gave u 2 solutions. I'm not going to write it for u but I guarantee both solutions work. And u can't buy my time, for any price.