The API does not give you the opportunity to set the blocksize, no.
You can increase the buffer by patching the constant on the function:
from poster.encode import MultipartParam
iter_encode = MultipartParam.iter_encode.im_func # function object
iter_encode.func_defaults = (8192,) # set new defaults, a 1-element tuple
The .func_defaults
parameter of functions can be replaced at will, just make sure you replace it with a tuple of equal length.