Adding things to a git bare repository

I know that if I have files in a bare repository, I can access them using git show HEAD:path/to/file.

But can I add new content to a bare repository without cloning and modifying a working tree?

if I add 1 file, only the 1 file is in that commit, aka we added something new and it's now set in stone

There's several convenient ways to add a single-file commit to the tip of the master branch in a bare repo.

So it appears i need to create a blob object, attach it to a tree, then attach that tree object to a commit.

All the ways to commit anything boil down to doing that, it's just a question of how well the convenience commands suit your purpose. git add creates a blob and makes an entry for it in the index; git commit does a git write-tree that adds any new trees for what's in the index, and a git commit-tree that adds a commit of the top-level resulting tree, and a git update-ref to keep HEAD up to date. Bare repos do have a HEAD commit, generally attached to (aka a symbolic ref for) a branch like master, . . .

So git's convenience commands are already doing almost exactly what you want. Especially with just the one file, this is going to be very easy.

Say for example your files appear in ~server/data/logs/, the bare repo you're using for distribution is at ~server/repo.git, you want the committed files to be at data/logs in the repo, and you always want to commit the latest logfile:

#!/bin/sh
cd ~server

# supply locations git ordinarily does on its own in working i.e. non-bare repos:

export GIT_DIR=$PWD/repo.git                  # bare repos don't have defaults for these
export GIT_WORK_TREE=$PWD                     # so supply some to suit our purpose
export GIT_INDEX_FILE=$GIT_DIR/scratch-index  # ...

# payload:  commit (only) the latest file in data/logs:

git read-tree --empty                       # make the index all pretty, and 
git add data/logs/`ls -1t data/logs|sed q`  # everything's ordinary from here - add and 
git commit -m'new logfile'                  # commit

git read-tree loads index entries from committed trees. It's what underlies checkout and merge and reset and probably some others I'm forgetting atm. Here, we just want an empty index to start, hence --empty.

use push/pull/remote to synchronize data while using a tool already available on every machine

You said "millions" of files over time, and if you don't want that full history distributed, rsync as I gather you already suspect might be a better bet. But -- one at a time, one new file per minute, it'll take two years to accumulate just one million. So, ?

Whatever, the above procedure is pretty efficiently extensible to any small-ish numbers of files per commit. For bulk work there are better ways.