Slow release should be fine. Don't overdo anything is a good rule.
At the same time, don't put all your stock into focusing solely on nitrogen. While nitrogen is necessary, the potassium (k) is actually the biggest aspect with hops for root growth and fighting disease in the long term.
Example: Look at all of the threads out here that talk about "what's happening to my leaves" etc. Most of these things are either disease, pests, over watering, stress, or too much chemical or natural fertilizer.
You aren't growing vegetables. You're working with bines, coming from roots. Either rhizomes or crowns need a strong established root system. That's where the K comes in. I use a rain barrel and a drip line and 0-0-21 liquid fertilizer for this purpose only. I am watchful keenly on not overdoing it.
I've had a good friend that was and is super organic about his vegetable garden and used a ton of chicken manure on his first hops (not vegetables) and ended up with some great big green leaves and hardly any yield because he was overloading the nitrogen and funny enough ended up feeding huge "bull shoots" (look that up) instead of supporting quality bines. The same thing happens with grape vines.
But, these hops again are "bines". They are roots first, then spikes to sidearms. When you read about winter kill, etc. it's all focused inherently to the roots. So take good care of the roots and you'll be happier in time with the bines.
Again, moderation and not always the big balance act towards nitrogen that is often touted as exactly necessary. Good top soil and compost mounded for long term protection and development is still fine. Let nature do the watering unless you see things being stressed or know that you are into drought.
That's my best advice and I haven't had any problems with people using slow release methods. In fact, it's often better than quick soluables. Caveat is always that if you do use liquid or pelletized forms to make sure you dilute or you will be sorry. Keep it hoppy!
