is there a (FLAC like) loseless compression coding format for EEG data?
For single channel with raw signal, the best option is Elias gamma, for single channel frequency signal, the best option is to use Range encoder. For multiple channels, this is still viable option to compress each channel and then compress them together interleaving signal (one sample from all electrodes, then another sample from all electrodes).

If you have multiple channels, there are inter-channel correlations, which should be used for compresion, standard method is to use DPCM or use DCT-IV for decorrelation (this is the very same transform like in JPEG) and then use arithmetic codding of any kind. For online setting - compress signal as it goes - this is very good idea.

Another standart approach is Karhunen-Loeve transform. Do not use autoregressive methods, use Elias gamma in the pipeline instead.
Another approach is to use DCT transform to remove redundancy of interchannels (decorrelation) and compress in minimal frequency form.  

If you need to store volumes of EEG data, my advice is to clear it from noises and store bands you are interested in, otherwise you store mainly hard to compress noise.