Is it legal to use a device's Bluetooth serial commands in one's project?
Disclaimer

I'm not a lawyer, but I am a tech-sector executive that deals with this issue regularly.

It's copyright, not DMCA

You are just figuring out (reverse-engineering) their API (e.g. Bluetooth command set) and then using it with your own program. Up until 2014, this was a widely recognized and protected safe practice. API's were considered to be ineligible for patent or copyright protections.

That all changed with Oracle's victory in its case against Google (Oracle v. Google). The courts ruled in Oracle's favor effectively reversing over 50 years of precedent and allowing API's to be copyrighted. An appeal to the US Supreme Court in 2015 was summarily rejected, forcing the case back to a lower court -- so still somewhat up in the air.

After losing on the "copyrights shouldn't apply to API's" argument, Google fell back to the Fair Use Doctrine as a defense and is expected to prevail. The problem is that now the burden shifts to the user to prove fair use. 

In your case, you might argue:


The use enables the creation of new technologies
You are using an insignificant percentage of the API
Your use is commentary on the API's design and application to wider purposes
Your use is educational for yourself or other developers


Practically... just do it

If you're not going to sell (or otherwise profit) from your project, then you are basically at near-zero risk. No one in their right mind would sue you over this. Worst-case, they'll just ask you to stop -- and if you do, the problem goes away.

Even if you are going to sell your project, since it helps sell their hardware, they will be strongly incentivised to leave you alone (assuming your sales don't cannibalize theirs).