Threw up a quick script:
https://codereview.chromium.org/2792693002/

Did you take a look to third_party/catapult/tracing/bin/strip_memory_infra_trace ?

The module doc says
"""Filters a big trace keeping only the last memory-infra dumps."""

And the CL description that introduced it (https://codereview.chromium.org/2184783002):

Author: primiano <primiano@chromium.org>
Date:   Wed Jul 27 13:38:12 2016

    Add tracing/bin/strip_memory_infra_trace

    We are running frequently in cases where traces w/ memory-infra
    are too big and fail to load, as their uncompressed size > 256M.
    While this is going to be fixed by bit.ly/TracingV2 we need a
    short term solution to open those traces, to deal with cases like
    crbug.com/623945.
    This script strips a trace with memory-infra keeping only the last
    two global dumps. It still guarantees that all processes for a dump
    are seen, and tries to maintain the detailed dumps, if any.


Which probably is what you want ?

Also, FYI, even without TracingV2 (proto stuff, punted) somebody is working on being able to import >256M by doing progressive parsing of the json in https://github.com/catapult-project/catapult/issues/2826