The interesting question about a smart belt is not whether you can fit electronics into one. It is whether a belt can do something a wrist physically cannot. If the answer is no, it is a watch with worse placement and it should not exist.
A belt has three properties a wrist does not: it is a closed loop around the torso, it holds a large area of skin contact, and it sits at your centre of mass. Everything below follows from one of those three, and every one of them is an addition to the design that has never been built.
The strap is an antenna
About a metre of conductive thread woven around the strap is a loop aerial in a class nothing on a wrist can reach. It is invisible, it adds no thickness, and it costs almost nothing. It is also what makes the two harder ideas below physically possible at all.
The open question is real: sharing one loop between Bluetooth, ultra-wideband and a low-frequency radio needs matching and filtering, and the loop's properties change with how tightly the belt is worn.
Your body is the key
Two electrode areas at the hips, thin foil integrated into the surface of the haptic zones, can put a microamp signal at kilohertz into you. Anything you then touch can tell that it is you. Touch a laptop and it unlocks. No card, no password, no reader.
This needs a large area of contact around the torso, which is the reason a watch cannot do it. It also needs a receiver on the other side, and that is the honest limit: the first version works with a laptop through a dongle. Touch anything and it knows you is a vision, not a feature, and we do not present it as one. Current limits into a human body are also not negotiable.
Your walk is the password
The waist is where movement laboratories put their sensors, because a wrist reads mostly arm noise. Gait is close to unique, so a six-axis sensor at the waist can keep quietly confirming that it is still you wearing the belt, and notice within roughly twenty steps if it is not.
Paired with body coupling, that combination means you never log in again: the belt knows it is you because of how you walk, and the thing you touch knows because of what your body is carrying. Neither half is much use alone.
What we do not know is the false-accept and false-reject rate on a real population, how it behaves in heeled shoes, on a bicycle, or running. And a gait signature is biometric data, so where the template lives, whether it leaves the phone, and how it is deleted are design decisions, not afterthoughts.
The number people actually watch
A strain gauge in the buckle's load path reads how tight the belt sits. It costs pennies, adds no thickness, and it fits in a part that already exists. What it produces is waist circumference over weeks and months, which is the number people follow more closely than weight when something is changing, and which no device measures on its own today.
The same signal carries respiration for free, without a single extra component. We have deliberately left that switched off. Turning it on is a decision about claims and regulation, not about hardware, and circumference, skin temperature and breathing can be presented as insight but never as diagnosis.
The cheapest test comes first
None of this exists as a prototype. So the order matters, and the first test is the strain gauge: one sensor, one belt, one afternoon. If there is no readable signal in the tension of a strap through clothing and while walking, then circumference and respiration both fall at once, and that is the cheapest possible way to find out.
Two other things deserve saying. Haptics stay two-zone even though we no longer think navigation is the point, because two zones and precise timing produce the sensation of one thing travelling around your waist rather than two separate taps. That is firmware, not hardware, and it is the cheapest item on the entire list.
And running a motion sensor continuously for gait, plus ranging, costs orders of magnitude more power than buzzing a motor. Until that power budget is written down, none of these functions can be promised. We would rather publish the gate than the promise.