The obvious solution to this isn't palatable due to the amount of work you'd need to do for each contact, but it's to change the way you use the name fields.
Specifically, do not add a second contact number for any contact you must know this information.
So, if you presently have me as a contact:
- Mike B - work, mobile, home
You would duplicate the contact (optionally linking them or storing them in different groups or even different iCloud accounts) and have three for me.
- @Home Mike B
- @Work Mike B
- @Mobile Mike B
Again, you have first and last name fields that iOS 6 shows. Furthermore, you would likely come up with a code to shorten @Home to a single character or digit. Finally, I see a legitimate use for emoji.
- 📞 Mike B
- 🏢 Mike B
- 🚁 Mike B
Based on your preference, you could pick color, shape, density to be very obvious or you could make it very subtle.
Now, this is an awful suggestion if you implement it for all contacts due to the multiplication of work to set it up and then maintain as things change. Worse, if all contacts are tagged, you will have that character or name bleed into other situations.
However, if you only had a few people you needed to know and only tagged their "alternate" numbers and made no other data be in that card, the chance you'd address a Mail envelope or share that contact card would be low and manageable.