I can create beatiful scatter plot with seaborns regplot, obtain the right level of transparency through the scatter_kws as in

sns.regplot(x='logAssets', y='logLTIFR', lowess=True, data=df, scatter_kws={'alpha':0.15}, line_kws={'color': 'red'})

and obtain this:

Is there an option in a seaborn pairplot to tweak transparency?

Ok I was very close to the solution. Seaborn pairplots have plot_kws that takes as arguments a dictionary of the kind of modifications you would do in a regplot. The following line is exactly what I needed:

g = sns.pairplot(df, kind='reg', plot_kws={'line_kws':{'color':'red'}, 'scatter_kws': {'alpha': 0.1}})

And this is the outcome:

If you don't do the regression but just the scatter plot (kind='scatter'), within plot keywords you don't have to do the division between line and scatter keywords:

g = sns.pairplot(df, kind='scatter', plot_kws={'alpha':0.1})