Sure.  But since this would be the first mirror from chromiumos/platform2 we'll have to add a new builder (rather than simply adding to the configuration of an existing builder).  So this will probably not be completed today.

no rush

Registered service account:
gsubtreed-chromiumos-platform2@chops-service-accounts.iam.gserviceaccount.com

Created a new group for it  && added as a member:
https://chromium-review.googlesource.com/admin/groups/5781,members

Created new repo
https://chromium.googlesource.com/chromiumos/platform2/libbrillo

Granted new group proper access to new repo
https://chromium-review.googlesource.com/admin/repos/chromiumos/platform2/libbrillo,access

Registering new builder with LUCI: https://chromium-review.googlesource.com/c/infra/infra/+/1200449

Registering account for Gerrit quota monitoring: http://cl/211172597

Finally, gsubtreed config:

$ git init
$ git remote add origin https://chromium.googlesource.com/chromiumos/platform2
$ vi config.json
$ cat config.json
{
  "enabled_paths": [
    "libbrillo"
  ],
  "enabled_refglobs": [
    "refs/heads/master"
  ]
}
$ git add config.json && git commit
$ git push origin HEAD:refs/gsubtreed-config/main

=> voila https://chromium.googlesource.com/chromiumos/platform2/+/refs/gsubtreed-config/main/config.json
(pinned https://chromium.googlesource.com/chromiumos/platform2/+/b0c4dfefaf2d8bbc1875c2c78cc3f987b30e0bc2/config.json)

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/390d00eb70805c83829aa1e474538dbbf155e2fa

commit 390d00eb70805c83829aa1e474538dbbf155e2fa
Author: Andrii Shyshkalov <tandrii@chromium.org>
Date: Sat Sep 01 02:09:12 2018

Add new gsubtreed builder for chromiumos/platform2.

R=smut

Bug:  877152 
Change-Id: I52fe22ad4296d7db87eea232cc1057b6c65d1107
Reviewed-on: https://chromium-review.googlesource.com/1200449
Reviewed-by: smut <smut@google.com>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

[modify] https://crrev.com/390d00eb70805c83829aa1e474538dbbf155e2fa/luci-milo.cfg
[modify] https://crrev.com/390d00eb70805c83829aa1e474538dbbf155e2fa/cr-buildbucket.cfg
[modify] https://crrev.com/390d00eb70805c83829aa1e474538dbbf155e2fa/luci-scheduler.cfg

Forgot to gsubtreed-chromiumos-platform2@chops-service-accounts.iam.gserviceaccount.com to https://chrome-infra-auth.appspot.com/auth/groups/project-infra-cron-task-accounts group.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/5e3b652bbb3b07dda603dffb2ac89cd3ef94d2d7

commit 5e3b652bbb3b07dda603dffb2ac89cd3ef94d2d7
Author: Andrii Shyshkalov <tandrii@chromium.org>
Date: Mon Sep 03 15:19:59 2018

Share available machines between less important gsubtreed builders.

R=nodir

Bug: 862060,  877152 
Change-Id: I3f8d5c11eec4e5b8c243a1d84225379c661181e5
Reviewed-on: https://chromium-review.googlesource.com/1200453
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>

[modify] https://crrev.com/5e3b652bbb3b07dda603dffb2ac89cd3ef94d2d7/cr-buildbucket.cfg

Repo has already been populated https://chromium.googlesource.com/chromiumos/platform2/libbrillo

Forgot a CL to add alerts if new gsubtreed isn't running: http://cl/211736045