I'd love to be able to update my vehicles by SMS, but currently there is no way to add tags via an SMS (I use tags to track the companies I buy gas from, the “level” of quality, and the reported octane rating.)
I'd love to see something like “This is a note this is a note [tag] [tag] [tag]” for SMS postings.
There just isn't much room to maneuver in SMS messages, and we can't make everything available there. I don't think brackets will do it because some people might like to use brackets in their notes.
I think your best bet is to note the info you want to use as tags in your note. Maybe even use the brackets like you have there. And then visit the website once in a while and add your tags to the fuel-ups later on. I know it's not as convenient as adding the tags directly, but I don't think there's a good way to add tags without complicating the SMS format quite a bit.