Here is an example of using 'QuadProg_Opts' option in svmtrain:
myoption = optimset('PrecondBandWidth',2);
% this is the ub parameter of quadprog I guess
svmStruct =svmtrain(train_data,train_label,'Kernel_Function','rbf','RBF_Sigma', 0.5,'QuadProg_Opts', myoptions);
But the C value seem to be relative to box constrain as used in the example of svmtrain:
svmStruct = svmtrain(data(train,:),groups(train),...
'showplot',true,'boxconstraint',1e6);