The KH-29L is a semi-active laser homing variant of the KH-29, it has no FLIR sensor to use to lock onto targets yet in-game it is capable of doing so. It's "irLock" value needs to be switched to 0.
However, such capability is available for the SU-34/SU-25 in the form of the KH-29D (FLIR guidance) variant. This is implementable by simply cloning the current KH-29 classname and creating a new one for the appropriate variant while switching the "irLock" value to 1 and the "laserLock" value to 0 for the KH-29D.
Sources: https://en.wikipedia.org/wiki/Kh-29 , http://www.deagel.com/Offensive-Weapons/Kh-29L_a001029001.aspx , http://www.enemyforces.net/missiles/kh_29l.htm , and http://www.armaco.bg/en/product/guided-missiles-c34/kh-29l-air-to-surface-missile-with-laser-active-radar-p560