Can I run a tee on wrought iron pipe at point of entry to house on outside of wall for lp gas wall heater. Is it safe?

Commonly done for gas lines to a grill. My house has one, steel pipe through house wall, T to grill, straight goes to gas FP. Safe if done properly. All pipe must be threaded and any connectors must be threaded or flare fitting, no compression. All threaded pipe and fittings must be made up with "approved pipe joint material, insoluble in the presence of fuel gas and applied to the male threads only". You use the term "wrought iron pipe". Some places interchange the term wrought and black steel piping. They are different. You also need a gas shutoff valve, properly installed.

"Accessible shutoff valves shall be installed in the fuel supply piping outside of each appliance and ahead of the union connection thereto, and in addition to any valve on the appliance. Shutoff valve shall be within 3 ft. of the appliance it serves and in the same room or space. UPC 1211.15."

Just make sure you make that connection on your side of the gas meter. As explained in above response, yes, you can T off from and run new branch. Few things to remember: The original pipe has to be sized right to accept new gas fixtures. If not, you will have to upgraded. Check for leaks before you activate your new installation. If pipe less than 6" from the ground, you have to use factory coated pipe, and all fitting and short nipples wrapped with 10mm wrap , treated with primer. Lastly, each gas fixture has to have its own shut off valve, before it reaches the fixtures

Lastly, and most importantly: you have to be licensed to work with gas

Good luck with your project !

Milo