The original Mirage Tank was supposed to be able disguise in a similar manner as spies.

Following instructions might help putting this feature back in.

Find the TankMakeupKit weapon.

Change it to this:

[TankMakeupKit]
Damage=1
ROF=100
Range=-2
Projectile=InvisibleAll
Speed=100
OmniFire=yes
FireOnce=yes
Warhead=TankSnapshot
RevealOnFire=no
TerrainFire=yes
Report=MirageTankDisguise
FireWhileMoving=no

Then, we need to edit the TankSnapshot warhead.

Change it to this:

[TankSnapshot]
Verses=0%,0%,0%,0%,0%,0%,0%,0%,0%,0%,0%
MakesDisguise=yes

Now to Mirage Tank rules.

Changes:
Primary=TankMakeupKit
Secondary=MirageGun
RadarVisible=no
RadarInvisible=yes

Doing this also allows the Mirage Tank get a better weapon when at elite status (don't forget that it is invisible on radar too!).