If a woman does not reach the climax at the time when the husband does, is it morally permissible for her to be stimulated until she achieves it? If a woman does not reach the climax in the marital act at the time when the husband achieves it (he achieves it before she does), is it morally permissible for her to be stimulated (by herself or her husband) until she achieves it? I am interested in the Catholic viewpoint.
Yes.

St. Alphonsus addresses this question in Theologia Moralis vol. 1, l. 6, n. 919 (pp. 675-6):


  An autem, si vir se retrahat post seminationem, sed ante seminationem mulieris, possit ipsa statim tactibus se excitare, ut seminet?Whether, if the man pulls out after insemination, but before the insemination of the woman, she can still excite herself with touches, that she inseminate?


(In this context, "inseminate" means "ejaculate" for a male or "be wet" for a female; it could, perhaps, also be translated as "orgasm.")

The moralists who thought she couldn't didn't realize the "semen mulieris" ["female seed" or mucus] "est necessarium ad generationem" ["is necessary for conception"]. But most moral theologians agree it is permissible.

St. Alphonsus concludes that, because the female is generally not as warm (aroused) as the male, the female can arouse herself by touching herself even after the male's insemination:


  Ratio, tum quia seminatio mulieris pertinet ad complendum actum coniugalem, qui consistit in seminatione utriusque coniugis, unde sicut potest uxor tactibus se præparare ad copulam, ita etiam potest actum copulae perficere; […] Omnes autem concedunt uxoribus quae frigidioris sunt naturae, posse tactibus se excitare ante copulam, ut seminent in congressu maritali statim habendo.The reason is that the woman's insemination pertains to the completion of the conjugal act, which consists in the insemination of both spouses; thus, as the woman can touch herself in preparation for copulation, so also can she to perfect/complete the act of copulation: […] All [moral theologians] concede that women, who are naturally more frigid, can excite themselves with touches before copulation so that they inseminate while having marital intercourse.


(adapted from this comment)

The act is complete/perfected not when the male inseminates, but when both spouses inseminate.

Also, even if a new act immediately follows a completed act, the spouses can, without mortally sinning, mutually decide to cease, provided there is no danger of ejaculation.

ibid. n. 918 (p. 674):


  Quaer. 3. An peccent mortaliter coniuges, si incœpta copula cohibeant seminationem. Respondetur: si coniuges ambo in hoc consentiunt, nec adsit periculum seminandi extra vas, id per se loquendo non est mortale, illa enim penetratio vasis fœminei tunc reputatur instar tactus verendorum, qui inter coniuges permittitur, vel saltem non est mortalis, secluso periculo pollutionis, ita communiterQuestion. 3. Whether spouses sin mortally if, copulation having begun, they hold back from insemination. Answer: If they both consent to this, there being no danger of insemination outside the vas (vagina), this is not per se mortal, for such penetration of the female vas is likened to venereal touches that are permitted between spouses, or it is at least not mortal, provided there is no danger of pollution (ejaculation outside of intercourse); this is the common opinion