Like umlaute explained, you could use a small script to check if there are any pull requests.
Here is a small python script i used a while ago to check for pull requests.
import urllib2
from StringIO import StringIO
import json
data = urllib2.urlopen("https://api.github.com/repos/<username>/<branch>/pulls").read()
io = StringIO(data)
jsondata = json.load(io)
for item in jsondata:
print "pull request :: " + item['title']
A different url would be needed to pull the data from stash. And the json data seems to have a slightly different structure.
for item in jsondata['values']:
Is the line that would need to be changed to get the pull requests from stash.