Why not add tracking features to upper stages? Has something like this ever been proposed?

Question

The identity of the object that will hit the Moon in early March is still uncertain. Some of the story is in Why did we think that the object was gonna hit the Moon March 4th 2022? Who's been tracking and predicting it so closely? and more can be found in this (temporary?) Project Pluto post Corrected identification of object about to hit the moon

It could have been an asteroid, was then assigned to the DSCOVR mission upper stage, and is now currently assigned to China's Chang'e-5 T1 though apparently there's some pushback from China as well as based on some info from 18SPCS.

• Space.com China says moonbound rocket stage was not from 2014 lunar mission: report
• Space.com Moon mystery: Who launched the rocket that will slam into the lunar far side?
• The Verge China casts doubt on origin of rocket debris about to slam into the Moon

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Question: Why not add tracking features to upper stages? Has something like this ever been proposed?

Since radar return signal strength drops as $1/r^4$ there needs to be other means to help in tracking and identification.

Perhaps the following will be helpful, as their signals drop as only $1/r^2$ for things near 1 AU

1. a little solar powered beacon gizmos that beep once per second on a specific frequency,
2. get some special paint with a unique reflection spectrum, say filled with some iridescent particles that use thin film interference to impose a fingerprint on the reflected sunlight's spectrum. That way any telescope that can pick them up can make a low-dispersion spectral analysis (a simple grating or GRISM somewhere in the optical path in front of the camera, no need for using an external spectrograph.

See:

• How were "microshutters" or other multiplexed or multi-object techniques first used in Astronomical spectroscopy?
• this answer to What equipment do I need to measure the spectrum of a star?

above: from here

See the FieldTestedSystems' YouTube video How to capture star spectra in your backyard

Answer 1

Such features have been added to an upper stage once at least, albeit for a very specialized purpose.

The RM-81 Agena

was an American rocket upper stage and satellite bus which was developed by Lockheed Corporation initially for the canceled WS-117L reconnaissance satellite program...[and] was later used as an upper stage, and an integrated component, for several programs, including Corona reconnaissance satellites and the Agena Target Vehicle used to demonstrate rendezvous and docking during Project Gemini.

(emphasis mine)

It was the latter (italicized) special use that involved the installation of "tracking features" on the upper stage.

Also added were strobe lights and an L-band radar transponder to aid in rendezvous operations.

Sources:

• Wikipedia Agena article https://en.wikipedia.org/wiki/RM-81_Agena
• Drew Ex Machina Gemini 9 article https://www.drewexmachina.com/2016/06/06/the-angry-alligator-the-snake-the-mission-of-gemini-9/