If it was that simple it would have been done by now.
It could possibly be made to work but for these reasons and possibly more (I don't have the cognitive ability to think this through properly at 3am) it hasn't been done.
- Those coin batteries come in a maximum of 3v, so you'd need at least 4 of them anyway (so there goes the space saving) or
- a voltage step up module like is used for deadrail.
- You'd need to prevent the track voltage from getting to the battery and prevent the battery from discharging (power being drawn from it) while track power is present. To do this you'd need some sort of circuit that switched the LM's power between track power and battery "Keep alive" accordingly when track power is lost and restored.
You'd need to do the third point as there is a reason they say not to recharge non-rechargeable batteries which is essentially what you'd be doing if you wired one in like a standard keep alive; even with a rechargeable battery you'd still need a proper charging circuit for it; and you'd want to prevent the battery from being used when track power is present as this is just a waste of the battery.
- Tim
