Is it possible to use DSP effects for format conversion ?
No. Currently it is only available with Audio CD burning.
(if not, then please consider to add this option to MC16)
A good feature request. This has been requested before and if I recall correctly Matt from JRiver has said that they are considering adding it. This is the correct time to remind them. The option should be available with regular conversions and also with MP3 CD burning and handheld syncs (when MC is set to convert). It would have many possible uses like volume leveling or EQ corrections.
Or is the only alternative to use the Disk Writer output option and transcode (convert) the resulting WAV files later ?
Yes. I have sometimes used the Disk Writer mode for creating cross-faded and volume leveled "DJ mixes".
Are there any alternative disk writer output plugins for MC15 (similar to Winamp output plugins to convert to other formats).
No that I am aware of.
EDIT
Matt posted while I was writing my reply...