The replacement metal tabs the OP used works.
I expect that the existing 'plastic' tabs are ABS. ABS is easy to solvent weld with readily available solvent cements (not the yellow stuff - clear is available if you search). Sheets of ABS can be obtained from Amazon and EBay vendors. If you want you can cut a little piece of ABS to replace the broken portion of the tab and cement in place. You will have to cut a backing piece (just like the metal pieces that the OP cut) to reenforce the tab and cement this in place. Drill holes for some small sheet metal screws to hold the reenforcement in place while the cement sets up. The screws can be removed later if you want. If you need to fill some gaps, take some ABS filings and mix with solvent cement to create an ABS slurry which can be spread into gaps - file and polish after it sets up if it needs to be pretty.
This isn't a better solution than what the OP came up with for the armrest tabs; but, it is probably slightly easier to cut and fit ABS than pieces of metal. The up-side is that you will have enough left over ABS to repair all the other tabs on the NSX in the future. I used this process to repair broken tabs on the trim pieces that hold the door liner in place where fabricating replacement metal tabs is not really an option (I tried) if you want it to look good.