That's great extra info to include.
Put yourself in the position of Mrs Blast at home:
- Blast said he'd be back on Sunday evening. Its 10:30 pm now and he's not back. What should I do ?

- Monday morning - he's still not back ! I don't want to seem a fool and call SAR if he's just running a bit late or decided to stay an extra night because he was too tired to drive.

-Oh look ! The note says to call the local ranger on "555 123 456" on Monday morning if he's not back. I'll call now ...

Writing some fairly specific instructions, even escalation instructions gives your family psychological "permission" to take action and to know what and when to do it.
If you are travelling with several people, give a home contact for each person, better than Mrs Blast saying to the SAR “He was travelling with someone called Aussie he met on the internet …”

Provide the phone numbers of the relevant authorities and contact details of the other people you are travelling with to make it as easy as possible for people to raise the alarm, or at least start the process.

If necessary nominate one person to be the primary point for coordination: Mrs Blast to check with Mrs Aussie, then call the Ranger …

I also include details such as the car description and rego; this means that a Ranger or SAR can quickly spot it if they need to, and Mrs Blast has the info to hand.
If there are possible alternative routes or plans, include them too eg “we may divert via big mountain if we’re ahead of scheule”, or “if the weather is bad, we’ll try and head to the town” … you get the picture.