Cheers to @Kache for mentioning what should have been obvious: just base64 encode to send plaintext.

For future reference, this is how I did so.

Client-side:

tag   = base64.b64encode(tag)
nonce = base64.b64encode(nonce)
# tag and nonce are packed into the payload dictionary
requests.post(url, json=payload, verify="myShnazzyCertificate.pem")

Server-side:

payload = flask.request.json

# tag and nonce are unpacked from the above payload dictionary and then
tag        = base64.b64decode(tag)                                              
nonce      = base64.b64decode(nonce)

# ... tag and nonce are used successfully to decrypt data