Pure AdLib based formats will have a very low priority because there are already better tools and libraries to handle those.

(OpenMPT uses RAD2's OPL3 emulator, by the way.)

There is also Adlib Tracker II located at http://www.adlibtracker.net

It is freeware with the source code (in Pascal though) available as well. Not sure how interesting it might be for you, but the following formats are supported.

AT2 loads the following song formats:

A2M (AT2)
A2P (AT2) [pattern]
A2T (AT2) [tiny module]
AMD (Amusic)
CFF (BoomTracker 4.0)
DFM (Digital-FM)
FMK (FM-Kingtracker)
HSC (HSC AdLib Composer / HSC-Tracker)
MTK (MPU-401 trakker)
RAD (Reality AdLib Tracker) {ver.1}
S3M (Scream Tracker 3.x)
SAT (Surprise! AdLib Tracker) {ver.1,5,6}
SA2 (Surprise! AdLib Tracker 2.0) {ver.8,9}
XMS (XMS-Tracker)

Loads the following instrument formats:

A2I (AT2)
A2F (AT2) [w/fm-register macro]
CIF (BoomTracker 4.0)
FIN (FM-Kingtracker)
INS (HSC-Tracker/RAD-Tracker, SAdT, Amusic/AdLib instrument)
SBI (Creative Labs FM instrument)
SGI (Sound Generator 3.0)

Loads the following bank formats:

A2B (AT2)
A2W (AT2) [w/macros]
BNK (AdLib instrument bank) {ver.1.0}
FIB (FM-Kingtracker)
IBK (Creative Labs FM instrument bank)

While sometimes taking a peek at how other trackers handle certain things, it doesn't help us immediately - you cannot just take one tracker's source and put it in another. The problem is not writing the loaders. The problem is rather that AdLib trackers will generally be distinct from what OpenMPT already does, and in particular for RAD2 this means for example "how to treat the extra two operators of OPL3 instruments". Stuff like this means a lot of work that in turn makes the engine and editor more complex.