Suggested maintenance for rear shock between service intervals? Typical rear shock has a suggested service interval around 150 hours or a year. 

What is a suggested maintenance between those service intervals? Just oil the stanchion from outside? Any technique to do that because it is harder to pass the oil by the stanchion than with a fork? 

Is the suggested interval a rule which should not be negotiated much or is it rather a recommendation which can be prolonged in case of less dirty rides (mud/wet).
It really depends on 


the conditions you ride in - wet and muddy or very fine dust may require more frequent tear downs.
whether or not you use a shock protector e.g a Lizard Skin 


IMO use one if you can but your frame may not allow a protector to fit in.


the age of the shock and the condition of the wiper seals. 


Again if the wiper seal is letting dust and water through the inner grease will become contaminated and useless.

Im using a Lizard Skin and I pull apart my current Fox about every 6 months which is often enough for me as the inner grease seems a little dirty by then. I'm typically riding 3-8 hours per week in summer and 1-2 hours in winter.

Greasing the outside will do nothing other than lubricate the wiper seals slightly.

To maintain you need to 


Clean all the dirt off the exterior of the shock. Give it a scrub in the sink if need be. 
Depressurise the shock.
No really, depressurise! Open up the air can. I can do this by hand but you might need a strap wrench for your shock model. It should have normal screw polarity so anti-clockwise to wind off. Be careful of how you do this so you don't break off adjustment levers.
Clean away all the old grease and any dirt on the wiper seal surfaces.
Place new Shock Grease on the inner O rings, damper shaft and wiper seal. Go easy here as you don't want to block ports. Less is more. Buy a tube of real shock grease. It's expensive but lasts forever.
Screw the air can back on. Again be careful as the can is made of aluminium and has fine threads which you don't want to cross. It's not going to be really easy as the negative chamber will seal up and resist compression.


Watch the procedure in this video: