Just tried the new functionality for machine readable output of "procstat" and found two issues with non standard compliant output generated for JSON and XML formats. Suppose that this is an issue with "libxo" library, not "procstat" but didn't dig further.
For JSON output hexadecimal numbers are printed as normal numbers while JSON standard says that numbers can be only decimal digits, in my opinion they needs to be printed as strings. Here is example output showing the issue:

# procstat --libxo:PJ -v 841
{
  "__version": "1",
  "procstat": {
    "vm": {
      "841": {
        "process_id": 841,
        "vm": [
          {
            "kve_start": 0x400000,
            "kve_end": 0x8f9000,
...


For XML output PID is used as name the element while XML element name must start with letter or underscore. Here is an example:

<procstat __version="1">
  <vm>
    <841>
      <process_id>841</process_id>
      <vm>
...