Doesn't seem particularly reasonable to not incarcerate anyone before trial.
The vast majority of people on bail aren't mass murderers or armed robbers, they're petty criminals and drug addicts. If you think a shoplifter might reoffend while on bail, then make his bail conditional on staying away from the local mall. If you think a domestic abuser might reoffend while on bail, then make his bail conditional on having no contact with his spouse. If you think that someone might abscond, then make his bail conditional on GPS tagging or regularly signing on at a police station.
Bail conditions (with suitable monitoring) are fairer and more effective than cash bail.
The purpose of cash bail is to make sure the suspect shows up to court. Which--sure, can be replaced with monitoring, and then instead of having a bail bondsman pick up bail-jumpers at zero cost to the taxpayer, the taxpayer can pay the sheriff's department to round up bail-jumpers instead.
On the one hand, if we're gonna do surgery on someone to surveil them hopefully it would be reserved for people actually convicted of a violent crime. On the other hand, I'd very possibly want the surgery myself if I thought the alternative was spending months in jail awaiting trial.
I'm not in the medical field but I'm sure there's many places such as "closish" to an artery that trained doctors would have no issues getting at but a random joe with a switch blade may have issues with. I'd figure just detecting the temperature change and alerting the authorities would be easier though, then you can place it just like a standard animal tracker, e.g. a needle in their arm/back.
I doubt that being close would deter, it would just lead to more desperate people committing a form of suicide for freedom.