"real-time adjustable" implies you want to be able to switch the window in the middle of another window(ex. you have a gaussian window and half-way through a grain with a gaussian window you want the grain to switch to having a square or other type of window?)… but i’m not sure why you’d want that since that would introduce clicks and pops at certain points where the switch between envelope-shapes causes waveform discontinuity… in any case, if you want something like that, you could look at this thread which talks about generating the window without writing it to a buffer~ and if you did that you might be able to work in some smoothing functions for the switches in shape:
http://www.cycling74.com/forums/index.php?t=msg&goto=163225

otherwise, if you are looking to generate different windows for each grain/segment of audio(scaled according to length, and only choosing a different window-shape at the start of each grain/segment) then i’d recommend the old-fashioned way of writing an amp-window into a buffer~ and then reading that buffer~ back at different rates in order to scale the window according to playback-length(plus, just fill the buffer~ with a different waveform or have several buffers with different amp-window shapes and have different poly~ grains address the different buffer~s containing amp-windows to get as many window-function choices as you need).

here’s a patch(attached) that shows two windowing techniques in case it helps(one is similar to your mention of a wave~/buffer~ technique), you could easily modify it to include different amp-window-shapes. (also, check out the adsr~ object, when used in conjunction with poly~ it’s very effective; and if you like adsr~, check out Eric Lyon’s el.adsr~ object for impulse-triggering of envelopes).