+agable@ +ehmaldonado@ who may also have strong opinions on bot_update.

From my PoV, you can implement this yourself in your own recipe:
 * bot_update to checkout needed revision
 * for patch in patches:
      git fetch <patch.ref>
      git cherry-pick FETCH_HEAD

We also want to support patches in dependent repositories, e.g. catapult, v8, skia, angle. Doing it in our own recipe would work for us, as long as we can get dependent repositories to work as well.

Getting it work for deps is more work because (in order of difficulty):
a) you need to figure out the path where DEPS-ed in project lives based on Gerrit project name.
b) for main gclient project (aka chromium/src) you apply patches then do gclient sync. For DEPS-ed in projects, you probably need to:
  1. apply all patches to chromium/src
  2. gclient sync
  3. apply all patches other than chromium/src.
c) but b) doens't support recursive DEPS (ie if say catapult had its own DEPS. If you need that, it's even more work :(

This is something I believe bot_update should support.
In the meantime:
a) 'gclient revinfo --url <project_url>' might help:
$ gclient revinfo --url https://chromium.googlesource.com/angle/angle.git
src/third_party/angle: https://chromium.googlesource.com/angle/angle.git@888081d59aae6bb4b2322501d84bd645b8a8c6de
You can get the output in json format as well.
b) I think this should work:
1. Apply all patches and commit.
2. gclient sync
c) I think 'b)' above would support recursive DEPS.